Fix import dialog errors (on cancel) and fix import folder dialog backdrop

Fixes #3066

Ignores an error of cancelation on file import.

Also moved the same logic to import folder action which had gotten rearranged so that the backdrop no longer showed up.  Common method now returns the job future and lets the action handle it.

@devoncarew 
This is confusing. `jobStatus != null` seems to indicate that the job has finished, yet we `_waitForJob` in this case. What's happening here?