errors: restructure formatparse() to clarify conditions a bit
The `similar` list will be calculated only for
`error.UnknownIdentifier`. It was then printed only if `inst.location
is None`, which is true for that exception type, but it's an indirect
condition to rely on.
Also, it looked from the code like it could both report similarities
and print a hint. That would be a little awkward because the
similarity report looks similar to the hint (both are printed within
parentheses). I also added a `elif` to clarify that. I plan to
refactor this more coming patches so the similarity report actually is
a hint.
Differential Revision: https://phab.mercurial-scm.org/D9343
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.