Fix handling of {log_error, Code, Req, Reason} events (take 2)

Although the log_error/3 event in webmachine_log has a spec that
indicates that Req is a `#wm_reqdata{}` record, the webmachine code
actually sends `{webmachine_request, #wm_reqstate{}}`.

This patch adds helper functions wm_method, wm_path, and wm_notes to
deal with the expected form. These helpers return undefined if
unexpected data is received to avoid crashing the logger and also
handle receiving a #wm_reqdata{} record.

The error and info events are logged to error_logger and NOT sent to
the request log. A line for these requests will appear there via
finish_request/acces_log event.

Tests adjusted to verify nothing sent to the request log for these
events. The error_logger output can be viewed on console when running
the tests. Integrating logging output into tests is left for another
time.