Fix emscripten_set_*_callback() to return EMSCRIPTEN_RESULT_UNKNOWN_TARGET if the target is not known. (#18977)

* Fix issue jeffreylanters/react-unity-webgl#250 where calling emscripten_set_wheel_callback() on an event target that does not exist would result in the page throwing an exception. After the change, an error diagnostic with callstack is logged when in ASSERTIONS builds, and the result EMSCRIPTEN_RESULT_UNKNOWN_TARGET is returned.

* Update a couple of cDefs

Author: juj