Suppress conductor logging of expected exceptions

RPC methods may return an exception from the ConductorManager as an
expected means to inform the API of an error. Such expected exceptions
should not be logged within the conductor -- otherwise the log will be
overflowing and real (unexpected) exceptions will go unnoticed.

This patch adds a decorator to the RPC methods in ConductorManager to
suppress logging of expected exceptions, while still passing them back
to the API.

Co-Authored-By: David Shrewsbury <shrewsbury.dave@gmail.com>

Change-Id: Ic3107a9af6afeb2a9e772ca0ce0acc45dbc182cb
Closes-bug: #1291509
Merge "Suppress conductor logging of expected exceptions"