Readable error messages

[deniak]

From Bert's whishlist

Readable error messages, preferably a format close to what compilers
generate, so you can use them inside Emacs or some other IDE.

An example of better error messages could be as follows. Rewrite

"key": "error",
"extra": {
  "line": 5,
  "column": 143,
  "message": "The “profile” attribute on the “head” element is 
  obsolete. To declare which “meta” terms are used in the document, 
  instead register the names as meta extensions. To trigger specific UA 
  behaviors, use a “link” element instead."
},
"type": "validation.html"

as:

Overview.html:5:143: error: The “profile” attribute on the “head” 
element is obsolete. To declare which “meta” terms are used in the 
document, instead register the names as meta extensions. To trigger 
specific UA behaviors, use a “link” element instead.