cli, python: simplify cli configuration
    
    There's no need to setup token defaults both in copr-cli and
    python-copr.  Simply delegate the default "Fedora Copr" configuration to
    the python-copr logic.
    
    Make sure that we fail verbosely if user explicitly specifies
    a non-existing token file.
    
    Drop the complicated error message that we stopped printing out anyway
    (at least not in appropriate situations for which that message was
    originally designed).
    
    Fixes: #2253