errors: raise InputError on early parse error in dispatch
I didn't think this would have any effect on the tests, but it does
because the catching in `scmutil.callcatch()` still happens. That's
because `dispatch` passes in the function that includes the parsing as
an argument to that function.
I initially used `ConfigError` here but Matt Harbison convinced me to
use `InputError`. I think that makes sense since error is not in a
config file.
Differential Revision: https://phab.mercurial-scm.org/D9387
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