While creating schema for the add-on I used, I realized that my error
message was not precise enough.
For a big configuration such as in "Enhance main window", the message
"None is not of type 'string'" is almost useless.
So I added a path in the json config, to indicate where the error
is. And also which subschema is currently being used.
this works around the issue Pontoon has with saving translations with
trailing newlines, and makes it easier for translators to update in
the future, as the errors are now using markdown
