failing to send mail should print a helpfull error message

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/lib/python3/dist-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "./uffd/csrf/csrf.py", line 29, in decorator
    return func(*args, **kwargs)
  File "./uffd/user/views_user.py", line 91, in update
    send_passwordreset(user.loginname)
  File "./uffd/selfservice/views.py", line 145, in send_passwordreset
    send_mail(user.mail, msg)
  File "./uffd/selfservice/views.py", line 151, in send_mail
    server.login(current_app.config['MAIL_USERNAME'], current_app.config['MAIL_PASSWORD'])
  File "/usr/lib/python3.7/smtplib.py", line 730, in login
    raise last_exception
  File "/usr/lib/python3.7/smtplib.py", line 721, in login
    initial_response_ok=initial_response_ok)
  File "/usr/lib/python3.7/smtplib.py", line 642, in auth
    raise SMTPAuthenticationError(code, resp)
smtplib.SMTPAuthenticationError: (535, b'5.7.8 Error: authentication failed: authentication failure')

closed with commit 5300cd91