UI: Add warning if starting the output fails

This is mainly to give visual feedback to those affected by NVENC not
working with older driver versions. Currenlty obs fails silently which
could go unnoticed for users who are using hotkeys as well as confuse
users who are not trained to read their logs when issues occur.

Closes jp9000/obs-studio#788

UI/window-basic-main.cpp

@@ -3876,6 +3876,11 @@ void OBSBasic::StartStreaming()
 			sysTrayStream->setText(ui->streamButton->text());
 			sysTrayStream->setEnabled(true);
 		}
+
+		QMessageBox::critical(this,
+				QTStr("Output.StartStreamFailed"),
+				QTStr("Output.StartFailedGeneric"));
+		return;
 	}
 
 	bool recordWhenStreaming = config_get_bool(GetGlobalConfig(),
@@ -3887,7 +3892,6 @@ void OBSBasic::StartStreaming()
 		"BasicWindow", "ReplayBufferWhileStreaming");
 	if (replayBufferWhileStreaming)
 		StartReplayBuffer();
-
 }
 
 #ifdef _WIN32