[cta-admin] Handle user errors from CTA Frontend:

- XRootD/SSI/Protobuf errors or CTA Exceptions return error code 1
  (something went wrong with communication or in the frontend)

- User errors return error code 2 (user specified an invalid drive,
  vid, etc.)

- If text output, print error message on stderr and exit with code 2

- If --json output, print error message on stderr, print empty JSON
  array [] on stdout, and exit with code 2