Add more exact message when import already registred external tool

Scenario:

1. Open the "Application Settings" dialog.
2. Go to the "External Tools" page.
3. Open the "Custom tools" group.
4. Click "Import".
5. In browse dialog select some external tool 'Test'
   Current result: the tool is registred
6. Click "Import" again and select the same tool
   Expected result: message about already registred tool
   Actual result:

    Task {Import custom external tools configuration} finished with error: Subtask {Register custom external tool} is failed: Can't register a custom external tool 'Test'
   

   Note, that it is possible to read notification only after closing "Application settings" dialog.