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
8 jobs for !1000 with fix-blur-fractional-scaling in 5 minutes and 29 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #644925
check_commit_log

00:00:24

passed #644927
eslint

00:01:12

passed #644926
js_check

00:00:42

passed #644929
no_template_check

00:00:30

passed #644928
potfile_check

00:00:31

 
  Build
passed #644930
build

00:03:20

 
  Test
passed #644931
test

00:00:55

passed #644932
test-pot

00:00:55