Avoid spamming conductor logs with object exceptions

Conductor's logs should include tracebacks only when something
unexpected happened, which is why we have the client_exceptions()
decorator. The object_action() and object_class_action() methods
are used for direct remoting of object methods, and thus really
should forward *any* exception to the client.

This patch does that, and also adds missing tests for these two
methods to verify the normal and exception-wrapped behavior.

Closes-bug: #1233789
Closes-bug: #1084706
Change-Id: I505462fa429a6aa68e7b8a08ec2b704bf18d029c
(cherry picked from commit 3a5e1faee04671f2e88b28d805b191b480054254)
Merge "Avoid spamming conductor logs with object exceptions" into milestone-proposed