1119688: Improved error code usage in subman.

- Standardized most print & sys.exit calls with the system_exit
  abstraction.

- Standardized error code usage, using os.EX_* constants where appropriate:
  - EX_USAGE for improper usage of command options
  - EX_DATAERR for proper command usage with invalid provided data
  - EX_UNAVAILABLE for command options which are unsupported by the
    current server
  - EX_SOFTWARE for internal fatal errors
  - Others used somewhat liberally where seemingly appropriate

- Added error output to the status command:
  - 0 when the system is registered and no errors occur during the check
  - 1 when the system is not registered

- Updated the tests as necessary
Merge pull request #1044 from candlepin/crog/1119688

1119688: Improved error code usage in subman.