Improve return code and message handling in queue manager module

[stewmcl]

This has needed doing for a long time now. Essentially the way that the return code and messages are handled in the queue manager module is a mix of simply returning the information using the exit_module function manually and using the result dictionary to store the information and then return it.

This needs unifying under one method, and the way its implemented in the functions needs clearing up as well so it is as consistent as it can be between each function.

[JamRamPage]

#105 moves module.exit_json() to main().

However, do we also need more work to preserve the message output if queue_manager is used on multiple QMGRs?