errors: let each Abort subclass define its error code It's more flexible to have the error codes defined on the error types themselves. That way extensions can easily set their own exit code. It also means that we can reduce a bit of duplication betwen `scmutil.callcatch()` and `chgserver.chgcmdserver.validate()`. Differential Revision: https://phab.mercurial-scm.org/D10735