Tell the user they need to fork rust-lang/rust rather than 500ing

See https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/Who.20makes.20rollups.3F/near/215134758.

Author: camelid

homu/server.py

@@ -264,6 +264,8 @@ def callback():
 
 def rollup(user_gh, state, repo_label, repo_cfg, repo):
     user_repo = user_gh.repository(user_gh.user().login, repo.name)
+    if user_repo is None:
+        return 'You must have a fork of rust-lang/rust named rust under your user account.'  # noqa
     base_repo = user_gh.repository(repo.owner.login, repo.name)
 
     nums = state.get('nums', [])