JavaScriptCore: Fixed exception object so sourceURL (file name), line (number), and message can be recovered.

The current implementation only returns an error string. But this is insufficient for debugging (what file and line number did it fail at?).

As documented here:
http://parmanoir.com/Taming_JavascriptCore_within_and_without_WebView
converting the JSValueRef of string to an JSObjectRef (via JSValueToObject) will trigger JSCore into filling the "sourceURL" and "line" properties into the object so they can be inspected by the caller.

Additionally, JavaScriptCore has a "message" property which contains the message string. JSCore doesn't seem to be filling this in for us automatically, unlike "sourceURL" and "line". So this patch also fills that property in too.

Thanks to Brian Barnes for the detailed information about "sourceURL", "line", and "message".

Below is an example (derived from Brian Barnes's information) on how you typically use/extract these exception details.

void script_exception_to_string(JSContextRef js_context,JSValueRef exception_value_ref,char* return_error_string, int return_error_string_max_length)
{
	JSObjectRef exception_object;
	JSValueRef value_ref;
	JSStringRef jsstring_property_name = NULL;
	JSValueRef temporary_exception = NULL;
	JSStringRef js_return_string = NULL;
	size_t bytes_needed;
	char* c_result_string = NULL;
	exception_object = JSValueToObject(js_context, exception_value_ref, NULL);
	// source and line numbers

	strcpy(return_error_string,"[");

	jsstring_property_name = JSStringCreateWithUTF8CString("sourceURL");
	value_ref = JSObjectGetProperty(js_context, exception_object, jsstring_property_name, &temporary_exception);
	JSStringRelease(jsstring_property_name);
	js_return_string = JSValueToStringCopy(js_context, value_ref, NULL);
	bytes_needed = JSStringGetMaximumUTF8CStringSize(js_return_string);
	c_result_string = (char*)calloc(bytes_needed, sizeof(char));
	JSStringGetUTF8CString(js_return_string, c_result_string, bytes_needed);
	SDL_Log("c_result_string: %s\n", c_result_string);
	JSStringRelease(js_return_string);
	strncat(return_error_string, c_result_string, return_error_string_max_length-1);
	free(c_result_string);

	strncat(return_error_string, ":", return_error_string_max_length-1);

	jsstring_property_name = JSStringCreateWithUTF8CString("line");
	value_ref = JSObjectGetProperty(js_context, exception_object, jsstring_property_name, &temporary_exception);
	JSStringRelease(jsstring_property_name);
	js_return_string = JSValueToStringCopy(js_context, value_ref, NULL);
	bytes_needed = JSStringGetMaximumUTF8CStringSize(js_return_string);
	c_result_string = (char*)calloc(bytes_needed, sizeof(char));
	JSStringGetUTF8CString(js_return_string, c_result_string, bytes_needed);
	SDL_Log("c_result_string: %s\n", c_result_string);
	JSStringRelease(js_return_string);
	strncat(return_error_string, c_result_string, return_error_string_max_length-1);
	//SDL_Log("c_result_string: %s\n", c_result_string);
	free(c_result_string);

	strncat(return_error_string, "]", return_error_string_max_length-1);

	/* get message */
	jsstring_property_name = JSStringCreateWithUTF8CString("message");
	value_ref = JSObjectGetProperty(js_context, exception_object, jsstring_property_name, &temporary_exception);
	JSStringRelease(jsstring_property_name);
	if(NULL == value_ref)
	{
		strncat(return_error_string, "Unknown Error", return_error_string_max_length-1);
	}
	else
	{
		js_return_string = JSValueToStringCopy(js_context, value_ref, NULL);
		bytes_needed = JSStringGetMaximumUTF8CStringSize(js_return_string);
		c_result_string = (char*)calloc(bytes_needed, sizeof(char));
		JSStringGetUTF8CString(js_return_string, c_result_string, bytes_needed);
		SDL_Log("c_result_string: %s\n", c_result_string);
		JSStringRelease(js_return_string);
		strncat(return_error_string, c_result_string, return_error_string_max_length-1);
		//SDL_Log("c_result_string: %s\n", c_result_string);
		free(c_result_string);
	}
}

To use:
if(js_exception)
{
	char return_error_string[256];
	script_exception_to_string(js_context, js_exception, return_error_string, 256);
	SDL_Log("Compile error is %s", return_error_string);
}
JavaScriptCore: Returning NULL for wrapper functions that expect JSValueRef may crash program.

According to this:
http://parmanoir.com/Taming_JavascriptCore_within_and_without_WebView
Returning NULL instead of an actual JSValueRef for a return value of a function could lead to crashes. I think I have seen related weirdness in the past when I failed to return a proper type to JSCore which resulted in very hard to understand behavior.

So this patch changes those return NULLs to return JSValueMakeUndefined().

I thought about JSObjectMakeError, but I don't fully understand the intent of the Error object and can't find any relevant real world examples of it being used. However, everybody seems to be using JSValueMakeUndefined().

This patch should be low impact since this is only triggered on an error condition.
JavaScriptCore: Reverted 2 of the JSValueMakeUndefined replacements because those functions are tied to JSObjectRef instead of JSValueRef. The C compiler will allow this, but C++ will reject the conversion.

It is unclear what the correct handling is for JavaScriptCore. (Nobody bothers to document this in JSCore.) Unlike our other problem where we incorrectly assume JSObjectRef when the functions want JSValueRef, this time Apple is demanding the JSObjectRef. Like our other problem, I assume it is unsafe to try to convert Undefined into a JSObjectRef.

So reverting to NULL seems like the safer bet for this specific case. Perhaps the other alternative is to return an exception object or an error object. But I would like to see JSCore document this before trying.
JavaScriptCore: Improved code that uses JSObjectMakeError instead of JSValueToObject to create the exception object.

JSObjectMakeError automatically populates the "message" field, and possibly other fields I don't know about. This seems to be the most robust way to create an exception object.

Thanks to Brian Barnes again for the tip on JSObjectMakeError.