Add location of invalid docstring in warning messages

[gpetiot]

Now the invalid docstrings are not reported as warnings anymore but make ocamlformat exit with code 1. I made it the default behavior but we can also choose to hide it behind a new option. (outdated)

[Julow]

I wonder if this can break the workflow of some people ? For example, when enabling parse-docstrings on a project that contains a lot of comments that don't parse, every comments will now have to be fixed at the same time.
I suspect this breaks parse-wyc, we should keep the warning in this case.

[gpetiot]

I reverted some of these changes, so ocamlformat still doesn't fail on invalid docstrings, all the warnings are printed during the parsing phase instead of in the middle of Fmt_ast, and missing locations are added.

[Julow]

The cost is to parse each docstrings twice. It's not a huge cost but what are the advantages ?
Showing the location is nice. (grepping for comments is not the best UI)

[gpetiot]

True, it made more sense when we would fail on invalid docstrings at the beginning, well I find it slightly better to group the checks when parsing the file. We could memoize the parsed docstrings if the cost is too visible to users

[gpetiot]

I moved the checks back where they were to reduce the diff.

[Julow]

Looks good.