Fail fast in solveLocalEqualities
This patch makes us fail fast in TcSimplify.solveLocalEqualities,
and in TcHsType.tc_hs_sig_type, if there are insoluble constraints.
Previously we ploughed on even if there were insoluble constraints,
leading to a cascade of hard-to-understand type errors. Failing
eagerly is much better; hence a lot of testsuite error message
changes. Eg if we have
f :: [Maybe] -> blah f xs = e
then trying typecheck 'f x = e' with an utterly bogus type
is just asking for trouble.
I can't quite remember what provoked me to make this change,
but I think the error messages are notably improved, by
removing confusing clutter and focusing on the real error.
(cherry picked from commit 5c1f268e2744fab2d36e64c163858995451d7095)