python: use the built-in API error handler for GSSAPI
    
    ERROR: Failed to determine Copr username from authentication.
    Error: Operation requires API authentication. See the 'AUTHENTICATION'
    section in man copr-cli(1).
    Error: Can't log-in using GSSAPI: Valid GSSAPI authentication supplied
    for user 'coprteam', but this user doesn't exist in the Copr build
    system.  Please log-in using the web-UI (without GSSAPI) first.