extend link-name for better understandability

So there already is the rule `link-name` that checks if there is an accessible name for a link. This rule fails if the link contains an image with an empty / non existing alt attribute. The error message is always:

> Links must have discernible text

My proposal is, that we extend this message in case of an empty / non existing alt attribute with something like:

> The link contains an image with an empty alt attribute although it describes the link.