Fix exception deserialiation on Python 3

Builtin exceptions are now in the builtins module, the exceptions module has
been removed.

Fix also the unit test for tracebacks, Unicode representation doesn't start
with "u" prefix anymore (u'...').

Change-Id: I422be457e23066699950c9a3999878ecf65f1b3f
Merge "Fix exception deserialiation on Python 3"