REST API should use error codes whenever possible

[ElDeveloper]

Originally brought up in this PR's comment.

The idea is to use HTTP error codes when an error occurs, the current pattern being followed is to always return code 200 and pack an error message in the response.

[ElDeveloper]

Adding as a note that we should make sure we standardize on the structure of our responses (as long as it makes sense) for all types of plugins, see discussion here: #1694 (comment)