pass exception instance instead of string to Failure

This fixes a DeprecationWarning like this:

    /usr/lib64/python2.6/site-packages/twisted/internet/defer.py:267: DeprecationWarning: Don't pass strings (like 'Timeout') to failure.Failure (replacing with a DefaultException).
      fail = failure.Failure(fail)

Change-Id: I369988a0e44441399df032f6489a7d1578b05c71
Merge "pass exception instance instead of string to Failure" into develop