Report multiple errors

It would be nice if, when relevant, `ppx_custom_yojson` would embed the error instead of raising.

For instance, in:
```ocaml
let invalid1 =
Format.printf !"The time is %{invalid#invalid} and the timezone is %{invalid#too}." time timezone
```

Currently, only the first error (`string "invalid#invalid" should be of the form [...]`) is reported, Merlin can't find any information about the arity of the generated function.

I would like to use this issue for an Outreachy project (see https://github.com/ocaml-ppx/ppxlib/issues/389). If you are willing to accept contributions, I would help in the reviewing process!

Start by examining how ppx_custom_yojson handles invalid format specifications in the example from the issue. The payload names no files or tests, so first locate that error-handling path and existing diagnostics; done should include reporting both errors and preserving Merlin's ability to determine the generated function arity.

Written by the indexing model from the issue text.