[EH] Print message for uncaught exceptions

After emscripten-core#17979, when `ASSERTIONS` is set, uncaught exceptions carry stack
traces that are printed to the screen for debugging help. This adds
an exception message to the exception objects in addition to that.

The message it adds is produced by `__get_exception_message` function:
https://github.com/emscripten-core/emscripten/blob/f6c46570e3780e52050bf822a07b342ec4bdddbe/system/lib/libcxxabi/src/cxa_exception_emscripten.cpp#L75-L111

If an exception is a subclass of `std::exception`, it returns
`std::exception::what()`. If not, it just prints its type.

If an exception is uncaught and its type is a subclass of
`std::exception`, now we print the `what()` message along with the stack
trace when `ASSERTION` is set. In case our exception is
`std::runtime_error` and the message is "my exception", when uncaught,
this prints:
```
exiting due to exception: [object WebAssembly.Exception],Error:
std::runtime_error,my exception
    at __cxa_throw (wasm://wasm/009a7c9a:wasm-function[1551]:0x24367)
    ...
```

Fixes emscripten-core#6330.

Author: aheejin

emcc.py

@@ -2641,6 +2641,7 @@ def get_full_import_name(name):
   # using the JS API, which needs this C++ tag exported.
   if settings.ASSERTIONS and settings.WASM_EXCEPTIONS:
     settings.EXPORTED_FUNCTIONS += ['___cpp_exception']
+    settings.EXPORT_EXCEPTION_HANDLING_HELPERS = True;
 
   # Make `getExceptionMessage` and other necessary functions available for use.
   if settings.EXPORT_EXCEPTION_HANDLING_HELPERS:

src/library_exceptions.js

@@ -431,7 +431,7 @@ var LibraryExceptions = {
   // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Exception
   // In release builds, this function is not needed and the native
   // _Unwind_RaiseException in libunwind is used instead.
-  __throw_exception_with_stack_trace__deps: ['$getCppExceptionTag'],
+  __throw_exception_with_stack_trace__deps: ['$getCppExceptionTag', '$getExceptionMessage'],
   __throw_exception_with_stack_trace: function(ex) {
     var e = new WebAssembly.Exception(getCppExceptionTag(), [ex], {traceStack: true});
     // The generated stack trace will be in the form of:
@@ -446,6 +446,7 @@ var LibraryExceptions = {
     var arr = e.stack.split('\n');
     arr.splice(1,1);
     e.stack = arr.join('\n');
+    e.message = getExceptionMessage(e);
     throw e;
   },
 #endif

system/lib/libcxxabi/src/cxa_exception.cpp

@@ -256,7 +256,7 @@ exception.
 
 #if defined(__USING_WASM_EXCEPTIONS__) && !defined(NDEBUG)
 extern "C" {
-void __throw_exception_with_stack_trace(_Unwind_Exception*, bool);
+void __throw_exception_with_stack_trace(_Unwind_Exception*);
 } // extern "C"
 #endif
 
@@ -293,7 +293,7 @@ __cxa_throw(void *thrown_object, std::type_info *tinfo, void (*dest)(void *)) {
 #else
     // In debug mode, call a JS library function to use WebAssembly.Exception JS
     // API, which enables us to include stack traces
-    __throw_exception_with_stack_trace(&exception_header->unwindHeader, true);
+    __throw_exception_with_stack_trace(&exception_header->unwindHeader);
 #endif
 #else
     _Unwind_RaiseException(&exception_header->unwindHeader);