Catch and improve display of generic Parser.Error

[smondet]

Cf. #391

I tried adding to the menhir grammar but would get shift/reduce conflicts and/or would be incomplete.

Catching and improving the error message is what Menhir themselves do:
https://gitlab.inria.fr/fpottier/menhir/blob/master/src/stage2/Driver.ml

PR checklist

• New code has tests to catch future regressions
• Documentation is up-to-date
• CHANGES.md is up-to-date

[smondet]

 $ cat > /tmp/bug.atd <<EOF
type t = {
  type: string;
  world: float;
}
EOF
 $ dune exec atdgen/bin/ag_main.exe -- /tmp/bug.atd
File "/tmp/bug.atd", line 3, characters 2-6: Syntax error

When Menhir does the job itself it looks slightly different:

 $ cat > /tmp/bug.atd <<EOF
type x = int
type t = {
  different: case;
  type: string;
  world: float;
}
EOF
 $ dune exec atdgen/bin/ag_main.exe -- /tmp/bug.atd
File "/tmp/bug.atd", line 4, characters 2-6:
Expecting '}'

(type is not the first field here)

[mjambon]

Nice!

[mjambon]

Note that if you write Closes #391 on its own line in the PR description, GitHub will close the issue for you when this gets merged.