common: Allow undefined $gsu_errors.

common: Allow undefined $gsu_errors.

If this variable is undefined, and the nounset option is turned on
(set -u), gsu aborts early when setting up the gsu_error_txt array.

Fix this by using ${gsu_errors:-} instead of plain $gsu_errors to
allow for scripts without error codes.

Reported by Congmao Wang.