Load backup input filter if downloaded filter does not exist

[Apollys]

When the user clicks Import Downloaded Filter:

• If the downloaded filter exists, run import_downloaded_filter
• Else if the input filter exists, run load_input_filter
  • Display message to user Downloaded filter not found, used backup input filter instead.
• Otherwise, display error message: Neither downloaded filter nor backup input filter exist.

[Apollys]

This is low priority now, because we've handled this in the opposite direction.

Whenever a profile is selected, the input filter is loaded, or the downloaded filter if the input filter doesn't exist.

So the only time a user clicks "Import Downloaded Filter" should really be when they want to import a newly downloaded filter.