UI: Warn user if recording format unsupported

If the recording format is unsupported or doesn't support multiple
tracks, warn the user to check their settings.

obs/data/locale/en-US.ini

@@ -62,6 +62,10 @@ Output.ConnectFail.InvalidStream="Could not access the specified channel or stre
 Output.ConnectFail.Error="An unexpected error occurred when trying to connect to the server.  More information in the log file."
 Output.ConnectFail.Disconnected="Disconnected from server."
 
+# output recording-related messages
+Output.RecordFail.Title="Failed to start recording"
+Output.RecordFail.Unsupported="The output format is either unsupported or does not support more than one audio track.  Please check your settings and try again."
+
 # output recording messages
 Output.BadPath.Title="Bad File Path"
 Output.BadPath.Text="The configured file output path is invalid.  Please check your settings to confirm that a valid file path has been set."

obs/window-basic-main-outputs.cpp

@@ -35,8 +35,10 @@ static void OBSStartRecording(void *data, calldata_t *params)
 static void OBSStopRecording(void *data, calldata_t *params)
 {
 	BasicOutputHandler *output = static_cast<BasicOutputHandler*>(data);
+	int code = (int)calldata_int(params, "code");
 
-	QMetaObject::invokeMethod(output->main, "RecordingStop");
+	QMetaObject::invokeMethod(output->main,
+			"RecordingStop", Q_ARG(int, code));
 	output->activeRefs--;
 
 	UNUSED_PARAMETER(params);

obs/window-basic-main.cpp

@@ -2712,10 +2712,15 @@ void OBSBasic::RecordingStart()
 	ui->recordButton->setText(QTStr("Basic.Main.StopRecording"));
 }
 
-void OBSBasic::RecordingStop()
+void OBSBasic::RecordingStop(int code)
 {
 	ui->statusbar->RecordingStopped();
 	ui->recordButton->setText(QTStr("Basic.Main.StartRecording"));
+
+	if (code == OBS_OUTPUT_UNSUPPORTED)
+		QMessageBox::information(this,
+				QTStr("Output.RecordFail.Title"),
+				QTStr("Output.RecordFail.Unsupported"));
 }
 
 void OBSBasic::on_streamButton_clicked()

obs/window-basic-main.hpp

@@ -165,7 +165,7 @@ public slots:
 	void StopRecording();
 
 	void RecordingStart();
-	void RecordingStop();
+	void RecordingStop(int code);
 
 	void SaveProject();