Thumbnailer: Don't retry thumbnail generations

In case they fail, especially with a crash, we don't want to retry the
thumbnailing automatically.
We now always insert a failure record, before updating it with the path
to the generated thumbnail.
Fix #95
4 jobs for dont_retry_thumbnailing in 13 minutes and 17 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Test
passed #281783
amd64 debian
asan-ubsan

00:10:08

passed #281780
amd64 debian
linux

00:07:23

passed #281781
win32
win32

00:13:15

passed #281782
win64
win64

00:12:51