Use LOG.exception when logging exceptions
Use LOG.exception when logging exceptions

This replaces usage of log.error that manually formats the
exception. It doesn't remove uses of LOG.warning with traceback
formatting as that would change the log level.

Change-Id: Ib0ab3a60496e20277e1a619dd266cccb1f8ba5b5

openstack/mistral,StackStorm/mistral,StackStorm/mistral,openstack/mistral