Tweak exception to work better with js toolchain

[bobzhang]

The current exception objects are encoded as below:

  throw {
        RE_EXN_ID: "Assert_failure",
        _1: [
          "tscanf_test.ml",
          174,
          2
        ],
        Error: new Error()
      };

This gives us good stacktrace when it is uncaught, but it does not work well with existing js toolchains. For example, mocha will catch such exception, and check the stack property which does not exist (we hit similar internal issues before). I am proposing to tweak it to the below one:

throw (Object.assign( new Error,  {
        RE_EXN_ID: "Assert_failure", // needed for internal marker
        message : "Assert_failure", // to make default printing idiomatic
        _1: [
          "tscanf_test.ml",
          174,
          2
        ]
 })       

This data representation should work better with existing js toolset.

TODO: make sure nullary exception, e.g, Not_found work as expected.
Cons: it seems the payload _1 not printed by default in this case

[bobzhang]

related to #4506

[bobzhang]

For the record, such encoding seems to strike a good balance:

{ Error : .., get stack() {return this.Error.stack}, ....} 

1. the stack extraction is lazy
2. the shadowing is rare, when it happens, it will not change the semantics
3. easy to implement

[danielo515]

This is extremely important for real world usage scenarios. As much as I love rescript type system, errors will happen, and exceptions will be thrown. Making custom exceptions play well with the host language and its tooling is critical to build applications. Lost of tools rely on the (bad) habit of communication by throwing exceptions, and proper exception shape is critical.

By the way, why not just extend the error class as has been suggested on the linked issue?

[fhammerschmidt]

This has finally been implemented in #6611