Changed NotImplementedException to NotImplementedError
Changed NotImplementedException to NotImplementedError

It seems NotImplementedException does not exist. It must be a relic
from old Python...

Signed-off-by: Eygene Ryabinkin <9f2104e8f86374257270d0eb74ac82c1c7e1ca3c@codelabs.ru>

frioux/offlineimap,frioux/offlineimap