[BUGFIX] #9 - Error message wrongly raised: "Invalid extension Weeks"

During some "playing around with CVSAnalY" i ran into the bug which was reported by @dicortazar in #9 Error message wrongly raised: "Invalid extension Weeks".

Here is my proposal to solve this issue:

The error is reproducable, if you does not have `pysqlite2.dbapi2` installed.

With the current master, you got the error `Invalid extension Weeks`.
With this patch you will get `Invalid extension Weeks (No module named pysqlite2.dbapi2)`

Maybe this is not the best solution to this problem, but this are more information as the current error message.