Lack of error handling in the WASM backend

[CryZe]

The wasm backend has a bunch of unwraps where proper propagation of the errors should be happening.

[yoshuawuyts]

@CryZe I believe you've been working on a reference implementation? I'd love for us to do better error propagation; but going from wasm's errors to proper error types was quite tricky. Did you find a solution for this?