hgweb: catch util.Abort raised by addchangegroup Right now, if a pretxnchangegroup hook fails, we send some HTML error message to the client and the transaction is not rolled back (issue499). Catching util.Abort allows us to send a decent message to the client and for some reason makes the rollback complete. This patch is not perfect since it doesn't fix the reason why the transaction wasn't rolled back (maybe some circular references?). Also, the transaction is aborted only after we've sent the response back to the client and the "transaction aborted" message ends up in the logs of the web server.

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