wasm: websockets could use finer grained errors

Emscripten has a number of result codes:

which can be used to finer grained errors

#define EMSCRIPTEN_RESULT_SUCCESS 0
#define EMSCRIPTEN_RESULT_DEFERRED 1
#define EMSCRIPTEN_RESULT_NOT_SUPPORTED -1
#define EMSCRIPTEN_RESULT_FAILED_NOT_DEFERRED -2
#define EMSCRIPTEN_RESULT_INVALID_TARGET -3
#define EMSCRIPTEN_RESULT_UNKNOWN_TARGET -4
#define EMSCRIPTEN_RESULT_INVALID_PARAM -5
#define EMSCRIPTEN_RESULT_FAILED -6
#define EMSCRIPTEN_RESULT_NO_DATA -7
#define EMSCRIPTEN_RESULT_TIMED_OUT -8