530 idempotency parsing error message

[torhovland]

Fixes #530.

• Improved error message when idempotency fails due to invalid output in the first pass.

[Niols]

Then:

$ nix run github:tweag/topiary/cba1993 -- -l ocaml <<EOF
let id = if true then Fun.id else function x -> x
EOF
[2023-06-27T08:37:48Z ERROR topiary] Parsing error between line 1, column 46 and line 1, column 47

Now:

$ nix run github:tweag/topiary/a3a04b3 -- -l ocaml <<EOF
let id = if true then Fun.id else function x -> x
EOF
[2023-06-27T08:37:56Z ERROR topiary] The formatter produced invalid output and
    failed when trying to format twice (idempotence check).
    
    If this happened with the built-in query files, it is a bug. It would be
    helpful if you logged this error at
    https://github.com/tweag/topiary/issues/new?assignees=&labels=type%3A+bug&template=bug_report.md
    
    The following is the error received when running the second time, but note
    that any line and column numbers refer to the formatted code, not the
    original input. Run Topiary with the --skip-idempotence flag to see this
    invalid formatted code.
[2023-06-27T08:37:56Z ERROR topiary] Cause: Parsing error between line 1, column 46 and line 1, column 47

Looks great to me!