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
