make VA Server launcher check whether python/py exists and give an error otherwise

Source/VAPlugin/Private/VAServerLauncher.cpp

@@ -128,11 +128,38 @@ bool FVAServerLauncher::StartVAServerLauncher()
   FString LauncherScript = TEXT("VirtualAcousticsStarterServer.py");
   if (FPaths::FileExists(FPaths::Combine(LauncherScriptDir, LauncherScript)))
   {
-    FString command = "cd/d "+ LauncherScriptDir+" & start python " + LauncherScript;
-    system(TCHAR_TO_ANSI(*command));
+		FString CMDCommand = "cd/d " + LauncherScriptDir + " & ";
+
+		//check whether py or python exist
+		auto DoesCommandExist = [&](FString Command)
+		{
+			//this checks whether a given command returns a result
+			FString TmpCmdResultFile = "tmpPyVersion.txt";
+			TmpCmdResultFile = FPaths::Combine(LauncherScriptDir, TmpCmdResultFile);
+			Command = Command + " >> " + TmpCmdResultFile;
+			system(TCHAR_TO_ANSI(*Command));
+			FString Result;
+			FFileHelper::LoadFileToString(Result, *TmpCmdResultFile);
+			IFileManager::Get().Delete(*TmpCmdResultFile);
+			return !Result.IsEmpty();
+		};
+
+		bool bPyExists = DoesCommandExist("py --version");
+		bool bPythonExists = DoesCommandExist("python --version");
+
+		if(bPythonExists || bPyExists)
+		{
+			FString Command = CMDCommand + "start " + (bPythonExists?"python":"py") + " " + LauncherScript;
+			system(TCHAR_TO_ANSI(*Command));
 			return true;
 		}
 		else
+		{
+			FVAUtils::OpenMessageBox("VA Launcher cannot be started since neither \"py\" nor \"python\" can be found. If it is installed add it to PATH (and restart Visual Studio)", true);
+			return false;
+		}
+  }
+  else
   {
     FVAUtils::LogStuff("[FVAServerLauncher::StartVAServerLauncher] Unable to automatically start the launcher script, looked for "+LauncherScript+" at "+LauncherScriptDir+". If you want to use this convenience function change the VALauncher Path in the Engine/Virtual Acoustics(VA) section of the project settings. However, nothing bad will happen without.");
   }