Let middleware handle MailmanApiErrors
Let the middleware handle the generic MailmanApiError that is raised when Mailman core is unavailable. This is so that all error handling can happen in one place and we don't need to clutter the views with exception handling.
I deployed it to my production setup and it's working.
Note that this merge request fixes an important issue mentioned in #171 (closed) (the issue is about more, but the fix is good for now.)