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