WASM support for exceptions

[gbryant-arm]

By design, WASM can't be tied to a specific embedder. Moreover, languages/compilers have their own definition of what an exception is.
As a result, exceptions aren't supported in WASM directly, although a proposal to natively support zero-cost exceptions is in the works (https://github.com/WebAssembly/exception-handling).

A workaround consists in implementing exception handling in the embedder. Emscripten, for instance, implements C++ exception handling (setjmp and longjmp among other functions) in the JavaScript glue code. It comes with a significant overhead, as a shim gets inserted into every function that can throw.