Parsing errors on invalid templates are uncaught, unclear, and unhelpful

[shonfeder]

A minimal example:

let () =
  ignore (Mustache.of_string "{{.name}}")

Which crashes with

Fatal error: exception Mustache_parser.MenhirBasics.Error

A first-level fix would be to catch this error and raise something a bit more informative (e.g., Invalid_mustache_template). A better fix would be to identify the location of the syntax error.

I'm happy to help with the first part, at least, if that would be welcome.

[shonfeder]

When I've managed to trigger tracebacks, they look like this:

Uncaught exception:  
  
  Mustache_parser.MenhirBasics.Error

Raised at file "lib/mustache_parser.ml", line 347, characters 8-18
Called from file "lib/mustache.ml" (inlined), line 151, characters 18-49
Called from file "lib/mustache.ml", line 212, characters 31-44

[rgrinberg]

Proper error messages would definitely be welcome.

[gasche]

I added support for reporting of lexing and parsing errors in #47, and then more support for render-time errors in #51 (and some more on #52). I also propose to add reference outputs for various error messages in #55. (As mentioned in #55, the error messages I have proposed so far do a reasonable job when the mistake is in the template file, not when the mistake is in the JSON, but this issue is on invalid templates.)

The particular example given by @shonfeder, however, is trickier than most and still badly handled with all these PRs. I don't understand all the details, but I think that this is related to the (somewhat unorthodox) way the lexer is setup. (.name is not a valid section name, but . is.) What happens in practice is that the error is only noticed at the very end of the file: a location is shown, but it is not helpful.

[gasche]

The particular example in this issue, {{.name}}, is fixed by #56 which provides a good error location in this case (right after the dot, and it says '}}' expected.)