Implement discover import error detection on 2.6.

Change-Id: I619a3cfa671ba4f1c0b4450c3a297046ec70bff1
Backport fix for load_tests in package modules.

Discover was not executing load_tests at the package level, in
contrast to the docs which suggested it was possible. This is a
backport of the Python upstream proposed fix.

Change-Id: Ib952d5ce4ae1a3da0cd27dcf09315d20055d6b5a
Closes-Bug: http://bugs.python.org/issue16662