A custom tool is valid even if it's executable doesn't exist

Scenario

1. Open "External Tools" page in the "Application Settings" dialog.
2. Import the following tool: "test/general/_common_data/workflow/custom tools configs/relativePath.xml". The executable file of this tool does not exist.
   Current result: the tool was imported correctly, there are no notifications.
   Expected result:
   • the tools was imported
   • the non-existent path is set for the tool
   • there is warning sign on the icon near the tool
   • a warning notification appears at the right bottom corner of UGENE window, when the Application Settings is closed it is possible to read it, it should be the following:

     The executable file "tool_path" specified for the imported custom tool "tool_name" doesn't exist. Make sure to set up a valid executable file before you use the tool.