A previous commit added a helper function for sending JSON data back to
the client.
This commit makes use of it for homogenizing the current implementation.
It also renames the existing helper message to send JSON errors to
starts with "send" because the new helper starts with it and they
helpers are clearer with their name starting with it.
Change-Id: I53ee0b4ca33d677a8ccd366c9ba6d73f4f472247
Fix the Admin API endpoints when it response with a client error
response code.
The most of the client response error send a JSON body but the
`Content-Type` header wasn't set to the appropriated value. This commit
fixes them and it adds assertions to the tests to very the
`Content-Type` header.
Updates the README to briefly document about the format of the client
errors endpoints responses.
Change-Id: Ifaf2122def801701211438ce241046be1adc0e8c
