installer: Add traceback for exceptions in thread

Previously it was only reported that an exception happened in the
installer thread, but now traceback was logged. Now also a traceback is
logged to help debugging.

Change-Id: I957d165e552c2ed4d8629e3c1a49804c8f225563
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>