Fail if any group name is not resolved to an ID.

cloud/amazon/ec2.py

@@ -883,6 +883,9 @@ def create_instances(module, ec2, vpc, override_count=None):
                 grp_details = ec2.get_all_security_groups()
             if isinstance(group_name, basestring):
                 group_name = [group_name]
+            unmatched = list(set(group_name) - set([str(grp.name) for grp in grp_details]))
+            if len(unmatched) > 0:
+                module.fail_json(msg="the following group names are not valid: %s" % ','.join(unmatched))
             group_id = [ str(grp.id) for grp in grp_details if str(grp.name) in group_name ]
         # Now we try to lookup the group id testing if group exists.
         elif group_id: