PDI-7734 - EE Repository export with Rules: When it fails, no UI feedback is given and the file is incomplete

Added ability to have detailed report on export process results.
Refactored repository exporter to handle null monitor calls without additional check-if-null complexity.
Some minor bugs with UI dialogs fixed.
Removed redundant dialog asking 'Are you sure to export into xml' as xml export is one available option.
Junit tests added.