add message if there is nothing to import

[willdavis]

version: 0.6.0-rc.1

Steps:

• Start Sketchpacks App
• Import a sketchpack
• Import the same sketchpack

Results:
On the 2nd import, no intersitial window will display and no message will be sent to the user.
The experience leaves the user wondering if the import function failed or not.

Expected:
intersitial window appears and then dissapears.
maybe a notification that nothing was imported