Improve error message for fragile or changed docstrings

[jberdine]

For cases where ocamlformat cannot reformat docstrings, the error message should be informative to and actionable by the user.

[gpetiot]

So the message added in #481 is not informative enough?

[jberdine]

The current message is not bad, but it does not indicate what the problem is specifically, just that something in the docstring isn't right. It would be better to more precisely indicate exactly where in the docstring the problem is located.