n-api: tighten null-checking and clean up last error

We left out null-checks for many of the parameters passed to our APIs.
In particular, arguments of type `napi_value` were often accepted
without a null-check, even though they should never be null.

Additionally, many APIs simply returned `napi_ok` on success. This
leaves in place an error that may have occurred in a previous N-API
call. Others (those which perform `NAPI_PREAMBLE(env)` at the top)
OTOH explicitly clear the last error before proceeding. With this
modification all APIs explicitly clear the last error on success.

Fixes nodejs/abi-stable-node#227