Add docs for http error maps

A pattern emerging in the new http namespaces is the error map:

http.get($.url,  { errors: false })

You can use the error map for 3 different things:

• You can pass errors: false to disable all error handling
• You can pass errors: { 404: 'message' } to set a custom error message (or throw for a code <400)
• You can pass errors: { 404: false } to not throw for a particular code

Adaptor docs right now have a really hard time documenting this behaviour in-line in a single sentence. It's just too complex for the API reference.

We should add a section in the docsite for this, probably in the job writing guide, and link to it from all adaptors

Start in the docsite's job writing guide and inspect the existing adaptor documentation. Add a dedicated section explaining the three http error-map behaviors, then link to it from all adaptors as requested. Done means the shared guidance is available and each relevant adaptor points readers to it.