Fail to generate random bytes instead of falling back to rand()

Fail to generate random bytes instead of falling back to rand()

This is more robust against broken setups where we run out
of memory or cannot read /dev/urandom.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=90414
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>
[smcv: document @error]
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>