I am having the same error - using 1.6.4 with Forge 9.11.1.965 and the python exception is 'HTTPError' is not defined in fml.py, line 1208. I'm on an Mac, 10.9.2 Mavericks.
If you worked around it I'd be interested.
Thanks, Matthew
UPDATE after some hacking
I'm getting a 403 when trying to download http://repo.maven.apache.org/maven2/org/scala-lang/scala-compiler/2.10.2/scala-compiler-2.10.2.jar; I'll see what else I can figure out.