prepare.py: return custom error code when working directory already exist so...

prepare.py: return custom error code when working directory already exist so that we can retrieve it easily

prepare.py

@@ -169,7 +169,7 @@ def run(target, debug, latest, list_cmake_variables, force_build, additional_arg
 		if force_build is False:
 			print("Working directory {} already exists. Please remove it (option -C or -c) before re-executing CMake "
 				"to avoid conflicts between executions.".format(target.cmake_dir))
-			return 1
+			return 51
 	else:
 		os.makedirs(target.abs_cmake_dir)