Currently, mod_atom handles client protocol errors with ap_log_error.  This
is wrong.  It should send a text/plain response body back with a
human-readable explanation of what went wrong.

httpd doesn't make this terribly easy, but mod_dav has kind of a hack that
apparently allows this to be done.

Don't forget to use apr_strerror on the apr_status to enhance
human-readability.