Chain exception re-raise in map_exceptions

[zanieb]

Preserves the exception chain instead of displaying "During handling of the above exception, another exception occurred" in the traceback.

Related to #677

[zanieb]

I'd like to get a few concrete before / after examples for this still.

[tomchristie]

Makes sense, yup.

Thanks 😌

I'd like to get a few concrete before / after examples for this still

I verified this with the following...

before:

>>> import httpcore
>>> httpcore.request("GET", "https://blahblahblahnope/")
Traceback (most recent call last):
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_exceptions.py", line 10, in map_exceptions
    yield
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/backends/sync.py", line 94, in connect_tcp
    sock = socket.create_connection(
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/socket.py", line 826, in create_connection
    for res in getaddrinfo(host, port, 0, SOCK_STREAM):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/socket.py", line 961, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
socket.gaierror: [Errno 8] nodename nor servname provided, or not known

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_api.py", line 39, in request
    return pool.request(
           ^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/interfaces.py", line 43, in request
    response = self.handle_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection_pool.py", line 253, in handle_request
    raise exc
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection_pool.py", line 237, in handle_request
    response = connection.handle_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 86, in handle_request
    raise exc
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 63, in handle_request
    stream = self._connect(request)
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 111, in _connect
    stream = self._network_backend.connect_tcp(**kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/backends/sync.py", line 93, in connect_tcp
    with map_exceptions(exc_map):
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/contextlib.py", line 155, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_exceptions.py", line 14, in map_exceptions
    raise to_exc(exc)
httpcore.ConnectError: [Errno 8] nodename nor servname provided, or not known

after:

>>> import httpcore
>>> httpcore.request("GET", "https://blahblahblahnope/")
Traceback (most recent call last):
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_exceptions.py", line 10, in map_exceptions
    yield
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/backends/sync.py", line 94, in connect_tcp
    sock = socket.create_connection(
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/socket.py", line 826, in create_connection
    for res in getaddrinfo(host, port, 0, SOCK_STREAM):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/socket.py", line 961, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
socket.gaierror: [Errno 8] nodename nor servname provided, or not known

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_api.py", line 39, in request
    return pool.request(
           ^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/interfaces.py", line 43, in request
    response = self.handle_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection_pool.py", line 253, in handle_request
    raise exc
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection_pool.py", line 237, in handle_request
    response = connection.handle_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 86, in handle_request
    raise exc
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 63, in handle_request
    stream = self._connect(request)
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 111, in _connect
    stream = self._network_backend.connect_tcp(**kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/backends/sync.py", line 93, in connect_tcp
    with map_exceptions(exc_map):
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/contextlib.py", line 155, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_exceptions.py", line 14, in map_exceptions
    raise to_exc(exc) from exc
httpcore.ConnectError: [Errno 8] nodename nor servname provided, or not known

And yes, the change of traceback from "During handling of the above exception, another exception occurred" to "The above exception was the direct cause of the following exception" makes complete sense.

I'll leave the merge to you.