[6.x] Ensure REST API authentication errors return consistent JSON

[daun]

• Always send a JSON response for unauthenticated REST API requests
• Include a WWW-Authenticate header, required by RFC 7235 to communicate authentication requirements
• FixesREST API: Unhelpful response on unauthenticated requests #15076

WWW-Authenticate: Bearer

{
"message": "Unauthenticated."
}

[daun]

Looks like the tests were cancelled by GitHub for some reason. Not sure why or how to rerun them.

[duncanmcclean]

Looks like the tests were cancelled by GitHub for some reason. Not sure why or how to rerun them.

It was probably GitHub - I was having issues with actions at the same time. I've just re-run them for you. 😄

[daun]

@duncanmcclean Thanks :)