terminal.py: do not stop searching for auto

If a terminal fails to spawn() we should continue looking.
gnome-terminal, in particular can be present but not start.

Signed-off-by: Joe Slater <joe.slater@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6e4babde)
Signed-off-by: Steve Sakoman <steve@sakoman.com>