Add a pformat() failure method and use it in the conductor

When the conductors engine raises a wrapped failure (which may
contain a single failure from a remote worker, or may contain many
failures if there were more than one such failures...) it is very
useful to show more than just the basic exception information (or at
least provide methods that can print out the information in a nice
manner).

This commit adds a pformat() failure method that formats the failure
in a nice and understandable manner and then uses it in the conductor
dispatching routine so that the conductors LOGs are much more useful.

Change-Id: I46afacf54c9b4cae885c76c09b51e61d71fe623a
Merge "Add a pformat() failure method and use it in the conductor"