Flag lexing/parsing failures properly

[LeventErkok]

This is related to https://github.com/phadej/cabal-extras/issues/131

In general, if docspec fails to parse a file for whatever reason, instead of skipping it, it should flag an error for that file. It doesn't have to be fatal: It can be just like any other doctest failure, but one that indicates it wasn't able to process the file for some reason.

Currently, it appears it simply skips the file, at least in certain circumstances such as the presence of unparseable unicode.