Sync execute() related exceptions with oslo

Cinder has its own ProcessExecutionError exception defined which
already exists in processutils.py. This was done to have minimal
editing when applying processutils.execute() to Cinder. As a result,
new instances are being created for each exception raised during
cli processing using execute()

As for the first step to remove the redundant excptions, this
patch syncs ProcessExecutionError and UnknownArgumentError
with that in processutils to ease the transition.

Change-Id: I88d9b86a0486f5d91fdcb664c99a9a2cd1392460
Merge "Sync execute() related exceptions with oslo"