Show a clear assertion failure if thread initialization fails (#8481)

Without this PR we'dt call _emscripten_futex_wake which is not set up, and fail there, hiding the real error.

Author: VirtualTim

src/worker.js

@@ -203,6 +203,12 @@ this.onmessage = function(e) {
         } else {
           Atomics.store(HEAPU32, (threadInfoStruct + 4 /*C_STRUCTS.pthread.threadExitCode*/ ) >> 2, (e instanceof {{{ makeAsmGlobalAccessInPthread('ExitStatus') }}}) ? e.status : -2 /*A custom entry specific to Emscripten denoting that the thread crashed.*/);
           Atomics.store(HEAPU32, (threadInfoStruct + 0 /*C_STRUCTS.pthread.threadStatus*/ ) >> 2, 1); // Mark the thread as no longer running.
+#if ASSERTIONS
+          if (typeof({{{ makeAsmGlobalAccessInPthread('_emscripten_futex_wake') }}}) !== "function") {
+            err("Thread Initialisation failed.");
+            throw e;
+          }
+#endif
           {{{ makeAsmGlobalAccessInPthread('_emscripten_futex_wake') }}}(threadInfoStruct + 0 /*C_STRUCTS.pthread.threadStatus*/, 0x7FFFFFFF/*INT_MAX*/); // Wake all threads waiting on this thread to finish.
           if (!(e instanceof {{{ makeAsmGlobalAccessInPthread('ExitStatus') }}})) throw e;
         }