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
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