Improve error message when srcset image doesn't load

[Daniel Bachhuber]

The current error message:

At a viewport of 1280×720 the image was displayed 560 pixels wide, but the closest provided image has a width of 1120 which is 50% (0.94 megapixels) off. The affected viewports are 300×169–3000×4000.

The suggested srcset widths exist, but the file paths are incorrect so the images don't load.

I was confused for a few seconds about why the linter was indicating the error. It would be helpful if the linter recognized the srcset values were already there, and warned me about the invalid files.

From #62