worker: raise exception instead of calling sys.exit() with child's code
When a worker process returns an error code, we would call
`sys.exit()` with that exit code on the main process. The `SystemExit`
exception would then get caught in `scmutil.callcatch()`, which would
return that error code. The comment there says "Commands shouldn't
sys.exit directly", which I agree with. This patch changes it so we
raise a specific exception when a worker fails so we can catch
instead. I think that means that `SystemExit` is now always an
internal error.
(I had earlier thought that this call to `sys.exit()` was from within
the child process until Matt Harbison made me look again, so thanks
for that!)
Differential Revision: https://phab.mercurial-scm.org/D9287
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