Prevent crash when trying to export all decks
Prevent crash when trying to export all decks

The crash was caused by the warning dialog, itself!  The issue was
that export is now run in a background thread, but GUI operations from
a background thread cause a crash.

AnkiModalNotifier is used in `anki_exporter_wrapper.py` and
`github_importer.py`.  The former is in a background thread, the
latter in the main thread.  In both cases, error messages are now
correctly displayed, without crashing.

Tested on Anki 2.1.40 and 2.1.43.

Stvad/CrowdAnki,Stvad/CrowdAnki,Stvad/CrowdAnki