Better native exception handling in threads

Currently when a thread is started we do :
```
try {
var result = Module['dynCall_ii'](e.data.start_routine, e.data.arg);
} catch (e) {
//handling
}
```

This works fine for JS errors, but it looks like Module.dynCall_ii() can also throw native error objects, which just get resolved to a pointer. This gives us unhelpful messages like:
`worker.js onmessage() captured an uncaught exception: {pointer}`
I'm sure there's a way to call into the native code and get some useful information out of the error, at a minimum something like:
```
static char* GetExceptionMessage(std::exception e) {return e.what()}
```
could provide valuable information.

I suspect that there is already something like this in Emscripten, is this the case?