Add helper function for C++ exception formatting

[hoodmane]

Resolves #16326
This adds a function that converts a C++ exception pointer into a message like:
CppException myexception: My exception happened
or:
CppException int: An object of type 'int' at address 15738976 was thrown as a C++ exception

You would use as follows:

try {
	Module._some_cpp_function(arg);
} catch(e){
	if(typeof e === "number"){
		// The number was a raw C++ exception
		const msg = Module.formatException(e);
		// ...
	}
}

TODO:

• Figure out how to make -sFORMAT_EXCEPTION_SUPPORT actually export this function
• Add a test
• Update docs?

[hoodmane]

@sbc100 I can't figure out how to force this function to be included from emcc.py. I want to add code like:

  if settings.FORMAT_EXCEPTION_SUPPORT:
    settings.REQUIRED_EXPORTS += ['formatException']

to force the library manager to put this in but I don't understand how.

[sbc100]

@sbc100 I can't figure out how to force this function to be included from emcc.py. I want to add code like:

  if settings.FORMAT_EXCEPTION_SUPPORT:
    settings.REQUIRED_EXPORTS += ['formatException']

to force the library manager to put this in but I don't understand how.

REQUIRED_EXPORTS only applies to native wasm function. I think want DEFAULT_LIBRARY_FUNCS_TO_INCLUDE and possible also EXPORTED_FUNCTIONS. See the include_and_export export helper in emcc.py

[sbc100]

lgtm % some last comments

[sbc100]

Add @with_both_eh_sjlj to this test?

[sbc100]

Then you can also remove DISABLE_EXCEPTION_CATCHING below.

[hoodmane]

It doesn't work with wasm-exceptions. I am not sure how to make it work with wasm-exceptions, if you like I can look into it. It would probably be entertaining. But I think it should be a separate PR.

[sbc100]

I see, can you open a bug about that so that we remember to get it fixed. I imagine @aheejin will want it to work for sure.

[hoodmane]

I opened #16380 which is where I got stuck in the wasm-exceptions case.

[sbc100]

lgtm with one final change

[kripken]

It would be good to add docs for this, perhaps around here:

https://emscripten.org/docs/porting/Debugging.html#handling-c-exceptions-from-javascript