Experiment with better error messaging.

[StevenDufresne]

This PR introduces an error readme file with information about each UI test.

Ideally, the readme only defines what the test does and leaves the actual information within the current handbooks.

Hoping to address:
WordPress/wordpress.org#26 (comment)

[carolinan]

This is better, it is still unclear:

• Which of the outputted messages are required according to theme directory requirements
• Which are recommendations
• What is the theme authors next step?

What is an unexpected link?

The menu check is incorrect. I don't think that belongs in this issue though:
Opening a sub menu on hover must not be required. Hovering is the least accessible way. It should not even be mentioned as recommended.

[StevenDufresne]

This is better, it is still unclear:

Which of the outputted messages are required according to theme directory requirements
Which are recommendations
What is the theme authors next step?

These are fair questions. I'll keep them top of mind.

The menu check is incorrect. I don't think that belongs in this issue though:
Opening a sub menu on hover must not be required. Hovering is the least accessible way. It should not even be mentioned as recommended.

Responded in the associated issue: #20.

What is an unexpected link?

https://make.wordpress.org/themes/handbook/review/required/#selling-credits-and-links

[StevenDufresne]

I'm going to merge this for now, but @carolinan points out some higher-level questions that will need answering. Roughly logged in #21.