add documentation about API responses

[jodeleeuw]

Create a documentation page that explains the errors listed in /functions/api-message.js. We can even add a link to the URL in the message for each error.

This also helps document the expected behavior of the API in general.

[jodeleeuw]

@jadeddelta this might be a good one for you?

[jadeddelta]

@jodeleeuw i'll definitely take a look at it!