errors: use exit code 10 for parse errors
Now that `ParseError`s raised while reading the config file has been
converted into `ConfigError`s, the remaining parse errors should all
be "input errors" (i.e. exit code 10), according to
https://www.mercurial-scm.org/wiki/ErrorCategoriesPlan.
Differential Revision: https://phab.mercurial-scm.org/D9332
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)