tool: Raise UsageError when used incorrectly

Commit e5e80540e8 added a new type of error, when user try to configure
non existing module. The test for this feature expect a SystemExit
exception, but the code was actualy raising a KeyError.

Seems that both the test and the code are wrong, so now a UsageError is
raised in this case.

Change-Id: I8e0528c84b41eee56c141d725c837cf48df5d186
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31735
Reviewed-by: mooli tayer <mtayer@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>