Do not hide SID resolver error in group-add-member

When group-add-member does not receive any resolved trusted domain
object SID, it raises an exception which hides any useful error
message passed by underlying resolution methods. Remove the exception
to reveal this error messages to user.

https://fedorahosted.org/freeipa/ticket/2997

ipalib/plugins/group.py

@@ -390,9 +390,6 @@ class group_add_member(LDAPAddMember):
                         failed_sids.append((sid, unicode(e)))
                     else:
                         sids.append(actual_sid)
-            if len(sids) == 0:
-                raise errors.ValidationError(name=_('external member'),
-                                             error=_('values are not recognized as valid SIDs from trusted domain'))
             restore = []
             if 'member' in failed and 'group' in failed['member']:
                 restore = failed['member']['group']