Prevent thumbnailing crashes when original is missing

Ghost User
2019-10-05T16:49:24+02:00

Closes #912

#### Previous behaviour
Steps to reproduce:

1. Remove an original image
2. Reload the page where the image should show
3. Crash

#### New behaviour
Steps to validate that it works:

1. Remove an original image
2. Reload the page where the image should show
3. No crash