Sync execute() related exceptions with oslo

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