[BACKLOG-186] - Need to provide user with a decent message when the repository is not available

Most of the changes are either cosmetic or minor

PRIMARY CHANGE: changed error message according to UX feedback
ADDITIONAL CHANGES:
slightly changed logic for process launch of Repository Explorer failure
added failure processing for JobDialog: extraTabs
change ctor of KettleRepositoryLostException to rely on super getMessage() and getLocalizedMessage()
checkstyle

@mattyb149 @brosander Colleagues would you mind to review?
@mbradbury2014 
@mtseu why are we using a string instead of a class reference? this won't survive a refactor