Cancelled export still creates a (broken) file

Steps to reproduce:

• open any larger file that will take a bit to export
• File > Export PNG image, choose to export at a high dpi
• Cancel the export

Use case:
accidentally click export with a wrong file name, and notice while it is still exporting, click on 'Cancel' and expect no file to be created.

What happened?

• exporting is aborted (good! thanks for the fix)
• an incomplete exported png file is being created (bad)

What should have happened?

• Option 1: clean up the partially exported file
• Option 2: export to a temporary file and rename that after export has finished (safer in principle, but might cause issues with non-local drives, not sure)

Inkscape Version and Operating System:

• Inkscape Version: 1.0beta2 (b38f114098, 2019-12-06)
• Operating System: Linux Mint 19.2