JIRA settings - investigate & improve configuration settings if possible

There are few things to consider:

1. Project key - it seems that we use this key only for configuration testing - if it is like that we should consider removing this key.

2. General error message when configuration is not working - we should be more specific - if eg. credentials are correct and the only problem is a wrong transition id or project key we should try to provide that information. It might not be possible with current implementation however we should be able to change it.

/cc @felipe_artur @smcgivern