Use sync error domain errors from network client

[stel]

Network client handles some sync-related (e.g. authentication) errors that should be reported by SyncManager directly.

Note that currently ROS never reports RLMSyncAuthErrorUserAlreadyExists, RLMSyncAuthErrorUserDoesNotExist codes and I'm not sure if that's changed. So probably this codes should be removed and InvalidCredentials could be moved to RLMSyncError enum.

@austinzheng, @jpsim let me know what you think.

[austinzheng]

Thanks for making this fix.

[jpsim]

Unfortunately, we can't rename RLMSyncAuthErrorInvalidCredential to RLMSyncAuthErrorInvalidCredentials, despite it being correct, because the sync APIs are now out of beta and so we're sticking to SemVer for them, and renaming this would break the public API.

[stel]

Sorry for a delay on this. I've updated the PR and added a corresponding test to check the reported error domain.

[jpsim]

Note that currently ROS never reports RLMSyncAuthErrorUserAlreadyExists, RLMSyncAuthErrorUserDoesNotExist codes and I'm not sure if that's changed. So probably this codes should be removed and InvalidCredentials could be moved to RLMSyncError enum.

I don't know if this comment is still accurate after your latest changes, but that's not quite right, as "user already exists" could be reported if the current user is an admin.

[stel]

It doesn't look like "user already exists" can be reported in any cases at all anymore (@mrackwitz please correct me if I'm wrong) while "user doesn't exists" can be reported when working with permissions, so it's not really a "SyncDomain" error.

I think if nothing is planned to be changed back on a server-side we should deprecate RLMSyncAuthErrorUserAlreadyExists and RLMSyncAuthErrorUserDoesNotExist.