Remove useless global vars / import

This removes a few import and global variables that are not used through
the code. That cleans things a little.

Change-Id: I7b30bb11e8ad3c2df01ca2107eff2444feed3fe2
Simplify common base exception prototype

It seems there's no gain in trying to be smarter and different from the
base Python Exception, so let's remove our custom code to be more
compatible and friendly with all Python versions.

Change-Id: I259783ef1f77c6661ea7dc2325605c8d6290b898
Merge "Remove useless global vars / import"