Uniform LocalDirectoryConnection.checkTarget() errors.

Hard to believe: there are uglier things the the True or Exception
pattern.

Change-Id: Icb8d2afd26efbd7ab228ccf20658485f776802de
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10150
Reviewed-by: Dan Kenigsberg <danken@redhat.com>