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 
Agreed, but this is the object we get back when the user cancels the chrome.fileSystem.chooseEntry file dialog. This method could use dartdoc to that effect.