Make exception syntax Python 3 compatible.

Translate all except and raise statements that are in the old form to
the Python 3 compatible format.

tests/letstest/multitester.py

@@ -141,7 +141,7 @@ def make_instance(instance_name,
     # give instance a name
     try:
         new_instance.create_tags(Tags=[{'Key': 'Name', 'Value': instance_name}])
-    except botocore.exceptions.ClientError, e:
+    except botocore.exceptions.ClientError as e:
         if "InvalidInstanceID.NotFound" in str(e):
             # This seems to be ephemeral... retry
             time.sleep(1)