Display the entries that failed when deleting with --continue.

We collected the failures but didn't report it back. This changes the
API of most delete commands so rather than returning a boolean it returns
a dict with the only current key as failed.

This also adds a new parameter flag, suppress_empty. This will try to
not print values that are empty if included. This makes the output of
the delete commands a bit prettier.

ticket 687

tests/test_xmlrpc/test_netgroup_plugin.py

@@ -1057,7 +1057,7 @@ class test_netgroup(Declarative):
             expected=dict(
                 value=netgroup1,
                 summary=u'Deleted netgroup "%s"' % netgroup1,
-                result=True,
+                result=dict(failed=u''),
             ),
         ),