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.
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