RD-10741 Improve error handling in the REST Client.

[miguelbranco80]

• Adds new exceptions for bad responses so that messages are clearly defined
• Centralizes automatic retries on 503 in a single method
• Remove accidentally passing bodies as exception messages for auth error

Note that this removes the "fundamentally broken" handleUnexpected method, which is still used by the Runner, so a small fix will be necessary there.