Some Shotwell results don't have a thumbnail

When results don't have a Shotwell generated thumbnail they use the generic "image" icon.
There should be a fallback on the actual image file, before trying to use this icon.

Test cases:

A) Deleting thumbnails:

1) Have photos imported in Shotwell
2) Ensure Shotwell is closed
3) rm -rf ~/.cache/shotwell/thumbs
4) Open the Photos lens

Expected:
Shotwell results are actual photos.

B) Deleting photos:

1) Have photos imported in Shotwell
2) Ensure Shotwell is closed
3) Delete photos
4) Open the Photos lens

Expected:
Shotwell results use Shotwell thumbnails.
Previewing them will inform you that the file is missing from your library and that you can fix it by opening Shotwell (which will display a "missing from library" category).

C) Deleting thumbnails and photos:

1) Have photos imported in Shotwell
2) Ensure Shotwell is closed
3) Delete photos
3) rm -rf ~/.cache/shotwell/thumbs
4) Open the Photos lens

Expected:
Shotwell results use "image" icons.
Previewing them will inform you that the file is missing from your library and that you can fix it by opening Shotwell (which will display a "missing from library" category).