Communicate exceptions through global memory

[alexcrichton]

Instead of allocating space on the stack and returning a pointer we
should be able to use a single global memory location to communicate
this error payload information. This shouldn't run into any reentrancy
issues since it's only stored just before returning to wasm and it's
always read just after returning from wasm.

[alexcrichton]

Note that this is part of the webidl work for #1524 where we ideally want to alter the ABIs of functions as little as possible. This is basically just a small refactoring which I think we want regardless and should make implementing #1524 a bit simpler.