copr: check for 'ok' in 'output' for json data (RhBug:1134378)

Traceback (most recent call last):
  File "/usr/bin/dnf", line 36, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 182, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 84, in main
    return _main(base, args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 134, in _main
    cli.run()
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 1072, in run
    return self.command.run(self.base.extcmds)
  File "/usr/lib/python2.7/site-packages/dnf-plugins/copr.py", line 147, in run
    while i < len(json_parse["repos"]):
KeyError: 'repos'

Now we introduce internal function to check 'output' field in json
output. If it is not 'ok', then raise exception with 'error' field in
json.

Reference: https://bugzilla.redhat.com/show_bug.cgi?id=1134378