blur-effect: Handle failure of background blitting gracefully

`paint_background` already provides a return value in case the blitting
of the framebuffer fails, handle that and fall back to only drawing the
actor in case something goes wrong.

GNOME/gnome-shell!1000
6 jobs for !1000 with fix-blur-fractional-scaling in 8 minutes and 51 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #591499
check_commit_log

00:00:22

passed #591501
eslint

00:01:04

passed #591500
js_check

00:00:42

 
  Build
passed #591502
build

00:06:01

 
  Test
passed #591503
test

00:00:40

passed #591504
test-pot

00:01:45