#51 Do not touch the target file if the export preconditions fail

[Ondřej Bouda]

When exporting an SCStudio diagram, a dialog prompts the user for the target file. If an existing file is selected and the "Overwrite?" dialog is confirmed, the export algorithm is started. The thing is that if it fails due to some unmet preconditions, the original file (selected for overwriting) is deleted while no new file is actually created. An expected behaviour is either to rewrite the original file with some contents, or not to touch it at all.

The export preconditions should be run even before prompting for the target file, thus, eliminating the mentioned odd.