member requests: "User already exists" should include link to existing user

[lukasjuhrich]

When finalizing a member request, sometimes the request errors with a flash message "user already exists". However, without a link to the conflicting user claimed to exist, that is harder to troubleshoot than necessary.

The reason for conflict should be displayed and conflicting user should be displayed with a link.

[lukasjuhrich]

This should work by attaching custom data (namely the debitorennummer in question) to the UserExistsException:

pycroft/pycroft/lib/user.py

Lines 1212 to 1213 in f25dc1f

	if user_swdd_person_id and not allow_existing:
	raise UserExistsException

…and then catching it to report it in detail.