Better handling of InconsistentMirrorError in aptmirror task

In #893 (closed) we had an issue with a repository that triggered the InconsistentMirrorError exception that the aptmirror task can trigger.

There are a few things that could have been better:

• that exception resulted in mails sent to the debusine administrators. Mails should not be required here since the failure is recorded in the work request log itself. It may mean that the exception is re-raised while it should not?
• the error message could be a bit more explicit about what the conflict is about, in that case it was that the Package entry in amd64 and armhf were not identical. Also the message logged in the work request does not included the name of the exception. You only get to see "akonadi-backend-mysql_1.13.0-2+deb8u2_all mismatch. Conflicting Packages entries:". See screenshot below.