Do not ask to register an email address if not supported by user store

The committer/author uploader validators issued an error message upon
failed validation that suggested to the user to register a matching
email address.

Some backing user stores do not support this. In this case, this
misleading message is surpressed now.

Change-Id: I5e7f54e0a9a3586205228cde41afae2f6346a90b
1 file changed