Fix touchpad backend initialization.

Libinput driver, even when unistalled was leaving 'Libinput Tapping
Enabled' atom active (xlsatoms | grep -i tap). Mutter/Clutter/Gtk+ were
causing this, as noted in https://bugzilla.redhat.com/show_bug.cgi?
id=1199825. This caused the non-existent libinput backend initialization
resulting in 'No touchpad found' error even when synaptics driver was
installed.
Fix this by checking backend instantiation errors.

BUG: 344958,349214
REVIEW: 124172
FIXED-IN: 5.4.0