user_external should log error message if authentication over imap does not work

[yennor]

Steps to reproduce

1. activate user_external app
2. change config.php to activate login over imap (but have an error in your configuration)
3. try to login

Expected behaviour

If it can not login over imap show a message what the error was!

Actual behaviour

It just shows a general purpose error message, that login was not successful.

It took me over an hour to find out, that the user_external app was properly enabled and even executed, just that the server certificate was wrong.
I managed to find that out by adding only adding a single line to imap.php

OCP\Util::writeLog('user_external', implode('|', imap_errors()), OCP\Util::ERROR);

[nickvergessen]

Please open a new issue in the repository of the app:
https://github.com/nextcloud/apps/issues