CSS checker enhancement request; output full details of each error’s location, including errors inside @imports

[CecilSWard]

In the CSS checker error messages should also (i) include the line number within @imported files if their content is also being scanned, (ii) the filename of the file where the error is found, and (iii) the line number of the current @import within the main file, as is being done currently.

The filename should be omitted if this is the main file.