errors: add config that lets user get more detailed exit codes This adds an experimental config that lets the user get more detailed exit codes. For example, there will be a specific error code for input/user errors. This is part of https://www.mercurial-scm.org/wiki/ErrorCategoriesPlan. I've made the config part of tweakdefaults. I've made the config enabled by default in tests. My reasoning is that we want to see that each specific error case gives the right exit code and we don't want to duplicate all error cases in the entire test suite. It also makes it easy to grep the `.t` files for `[255]` to find which cases we have left to fix. The logic for the current exit codes is quite simple, so I'm not too worried about regressions there. I've added a test case specifically for the "legacy" exit codes. I've set the detailed exit status only for the case of `InterventionRequired` and `SystemExit` for now (the cases where we currently return something other than 255), just to show that it works. Differential Revision: https://phab.mercurial-scm.org/D9238