Fixed refresh token validation error message covers too many cases.

[sun]

Problem

• The error message returned by the refresh token validation does not indicate why the refresh token is obsolete.

Details

• On our site, one of the mobile apps receives an error response when trying to regenerate the refresh token.
• We want to learn into which exact validation case the app runs into.

Proposed solution

1. Return a different error response for each of the validation cases.

Remaining Todos

• [x] Document the new codes in the readme files
• [x] Update .pot file — Skipping this because the .pot file is empty/malformed

[dominic-ks]

@sun looks good to me, though I couldn't actually seem to produce the jwt_auth_invalid_refresh_token error, if I changed the refresh token it just told me it was obsolete.