internal/v4: implement extra-info GET and PUT
Fix the tests failing due to the introduction of the extrainfo field and to the error refactoring.
Use the not found error representation in utils.
Rename put to handlePut as per review.
Merge pull request #88 from frankban/037-extra-info

Implement extra-info GET and PUT API endpoints.

This includes both extra-info and extra-info with key.
Also improve how not found errors are handled by the router and the endpoints.

This is mostly based on Roger's work. I just fixed the failing tests and other minors.