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_privilege_plugin.py

@@ -355,7 +355,7 @@ class test_privilege(Declarative):
             desc='Delete %r' % privilege1,
             command=('privilege_del', [privilege1], {}),
             expected=dict(
-                result=True,
+                result=dict(failed=u''),
                 value=privilege1,
                 summary=u'Deleted privilege "%s"' % privilege1,
             )