Provide more context on Cabal file parsing errors

[zzantares]

When ormolu fails to parse the Cabal file the user doesn't know what is the actual error as only the cabal filepath is reported. This makes the necessary changes to actually show what errors the cabal file parser encountered.

Before:

  Parsing this .cabal file failed:
    /path/to/file.cabal

After:

  Parsing this .cabal file failed:
    /path/to/file.cabal:0:0: Unsupported cabal-version 3.8. See https://github.com/haskell/cabal/issues/4899.

Note: Removing the Eq instance on OrmoluException was the easy way to not
having to implement an orphan Eq instance on PError. This does not have
any impact elsewhere as we're actually not making use of the Eq instance
on this exception type.

[mrkkrp]

As a consequence of c81320d, this actually constitutes now a breaking change 🤔

[amesgen]

As a consequence of c81320d, this actually constitutes now a breaking change 🤔

Yeah, maybe the breaking change becomes more "justifiable" when we also add the error code to OrmoluException in #983:

ormolu/src/Ormolu/Parser.hs

Lines 115 to 117 in 65539b9

	codeMsg = case diagnosticCode err of
	Just code -> "[" <> showOutputable code <> "] "
	Nothing -> ""

An option to prevent this in the future would be ofc to make OrmoluException opaque as most users probably just want to print it.

If we really want to, we can probably merge this and still maintain compatibility via some PatternSynonym dances (not sure if worth it).

---

In general, I think this change looks good, I think it is mostly useful when Ormolu uses a different cabal version than the installed cabal binary (as otherwise, you get the same error message when invoking cabal).

[zzantares]

Hey! apologies for the late response, I've applied the suggestions.

BTW if this is a breaking change and brings you guys much pain then feel free to skip this if it suits you; the only thing I really needed was to actually see what was the problem I was running into.

[mrkkrp]

I believe #983 will warrant Ormolu 0.6.0.0, and so we can merge this without any concerns about backwards compatibility.