tests/test-gendoc-ro.t

dispatch: move part of callcatch to scmutil

Per discussion at 39149b6036e6 [1], we need "callcatch" in worker.py. Move
it to scmutil.py to avoid cycles.

Note that dispatch's callcatch handles some additional high-level exceptions
related to config parsing, and commands. Moving them to scmutil will make
scmutil depend on "commands" or require "_formatparse" and "_getsimilar"
(and "difflib") to be moved as well. In the worker use-case, it is forked
when config and commands are fully loaded. So it should not care about those
exceptions.

[1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-August/087116.html

#require docutils gettext

  $ $TESTDIR/check-gendoc ro
  checking for parse errors