Issue a warning if user forgets to create wiki before running moin

[RogerHaase]

If user does this with quickinstall menu system, there is a warning, but it is based on a missing file directory. Can it be a moin storage / whoosh storage independent check (so that the real check is in the storage implementation, not in moin highlevel code)?

If user starts the built in server with "moin moin" there is a traceback.

I suspect there will be a traceback and 500 error response if moin were run as a wsgi app, but have not tested this.