errors: raise ConfigError on failure to parse config file This replaces two raises of `ParseError` by `ConfigError`, which makes it so we get the desired exit code when `ui.detailed-exit-code` is enabled. Because the exceptions include a location, I had to add that to `ConfigError` as well. I considered making `ConfigError` a subclass of `ParseError`, but it doesn't feel like it quite passes the "is-a" test. I used "config error: " as prefix for these errors instead of the previous "hg: parse error: ", which seems a little less accurate now (and, as I've said before, I don't know what the "hg: " part is supposed to signify anyway). I can easily be convinced to change the prefix to something else (including "abort: "). Some of the exceptions raised here mean that we fail to even load the `ui` object in the `dispatch` module. When that happens, we don't know to use detailed exit codes, so some tests (e.g. `test-hgrc.t`) still see exit code 255. I'll try to get back to that later. It should be possible to give detailed exit codes if at least part of the config can be read (e.g. when the system-wide one enables detailed exit codes and the user's config fails to parse). Differential Revision: https://phab.mercurial-scm.org/D9355