flatpak: propagate task errors in bootstrap

We can get into an infinite (albeit async via user-flow) state if the
downloads fail and pipeline reloads.

This only invalidates the pipeline if the task was successful.

Related #1430
2 jobs for master in 62 minutes and 34 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #1238396
flatpak
flatpak

00:56:24

 
  Deploy
passed #1238397
nightly

00:06:09