Improve the error if the tool wasn't imported because of the existing of the tool with the same ID

Scenario:

1. Download both attached files and put them into the same location.
2. Open the "External Tools" page the "Application Settings" dialog.
3. Import the "my_custom_tool.xml" tool
4. Import the "my_custom_tool.xml" tool again
   Current result: there is the error notification appears with the following message

   Can't register a custom external tool 'tool name'
   

   Expected result: there is the error notification appears with the following message

   Can't register a custom external tool 'tool name', because the tool with the same ID was already registered.