Show JS exception details for FATAL ERROR message

If ChakraShim fails to initialize, there is no clear information on what is going on. This happened with the latest Windows update.

Suggested change shows the details of JS exception (if any)

deps/chakrashim/src/jsrtutils.cc

@@ -866,12 +866,36 @@ void Unimplemented(const char * message) {
 }
 
 void Fatal(const char * format, ...) {
+  bool hasException;
+  JsErrorCode errorCode;
+  JsValueRef exceptionRef;
+  JsValueRef stackRef;
+  JsValueRef strErrorRef;
+  size_t stringLength;
+  const wchar_t* strError;
+
   va_list args;
   va_start(args, format);
   fprintf(stderr, "FATAL ERROR: ");
   vfprintf(stderr, format, args);
   va_end(args);
 
+  errorCode = JsHasException(&hasException);
+  if (!hasException || errorCode != JsNoError) {
+    if (errorCode != JsNoError)
+      fprintf(stderr, "\nImportant: While trying to check Javascript "
+        "exception, JsHasException has also failed.\n");
+    else
+      fprintf(stderr, "\nImportant: This didn't happen because of an "
+        "uncaught Javascript exception.\n");
+  }
+  else if (JsGetAndClearException(&exceptionRef) == JsNoError &&
+           GetProperty(exceptionRef, L"stack", &stackRef) == JsNoError &&
+           JsConvertValueToString(stackRef, &strErrorRef) == JsNoError  &&
+           JsStringToPointer(strErrorRef, &strError, &stringLength) == JsNoError) {
+    fwprintf(stderr, L"\n%s\n", strError);
+  }
+
 #ifdef DEBUG
   __debugbreak();
 #endif