exchange: raise error.Abort instead of ValueError Raising ValueError results in an uncaught exception and a traceback being printed. In the context of servers, it can result in an HTTP 500 and an exception being logged in the error log. I don't think this is proper behavior. The bundle2 code paths have a mechanism for translating an error.Abort into an error message reported to the clients. I think we should use that instead. This commit replaces some ValueError with Abort so that servers can error more gracefully. Differential Revision: https://phab.mercurial-scm.org/D5972