Improved clarity on error names

[joedolson]

The context checked varies on different tests. For example, 'Missing subheadings' is only a test of post content, but 'Incorrect heading order' is a full page text. I think it would improve usability if errors included clear indications of context checked in their names.

E.g.

Full page: Incorrect Heading Order
Post content: Missing subheadings

This would speed comprehension and handling of issues.

[SteveJonesDev]

@pattonwebz, I'm adding this to the v1.15.0 milestone as it somewhat relates to the landmark identification we're working on.