Use valid exception in start_shellinabox_console

Replace non-existing exception exception.ProcessExecutionError
with the correct exception from processutils.

Remove non-existing exception exception.UnknownArgumentError
because it was meant to be processutils.UnknownArgumentError
which isn't needed since it is only raised if there are
unknown arguments in kwargs, which there aren't in this case
(only 'check_exit_code' is there.)

Adds a unit test that checks for the ProcessExecutionError
exception.

Change-Id: I756f78bd8d8976a638169ee9db712a95dc664b58
Closes-Bug: #1356408
Merge "Use valid exception in start_shellinabox_console"