BUG: wsgi.error should be TextIO, not BytesIO in WSGI transport

[adriangb]

I found this from here: https://github.com/falconry/falcon/blob/e255bff9ae5a90d0cb3fe9af7c16917f18a92dc3/falcon/request.py#L1879

Based on the documentation at https://modwsgi.readthedocs.io/en/master/user-guides/debugging-techniques.html#apache-error-log-files, I believe Falcon is right and HTTPX is wrong.

This PR:

1. Adds a parameter so that we can test it and users can easily collect the logs in their tests if they want.
2. Changes the default from an inaccessible BytesIO (wrong type of stream, can't be accessed by user) to sys.stderr so that users (or pytest automatically) will capture the logs.

[tomchristie]

Neat little, addition, yup! Thanks!