better error message from using Archive pointing at nonexistent data

[sbenthall]

Trying to initialize an Archive object with a mailing list name that is invalid or not in the archive directory gets you this rather uninformative error message:

ArchiveWarning: Error while converting to datetime, despite coerce mode. The error message is: 'NoneType' object is not subscriptable

It would be better if it delivered a better error message.