This way, if the user has provided a valid, existing output in the list
of outputs, the focus & move workspace to output commands will not
report a misleading failure.
Side-effect is that the command code will try to execute a no-op e.g. by
moving the workspace to the output it already is on. But that's what the
user is actually requesting in this case and it shouldn't be a problem.
Fixes#4691
