trusted-firmware.inc: Catch failures on "Booting BL31" stage too

Issue was seen in LTS jobs:
https://tf.validation.linaro.org/scheduler/job/1645072

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I83d01f475ff4bd281883d81de2ca4b924b993d5a
(cherry picked from commit a59466755cae57a6d9d4878b868a88333c07aa4f)
1 file changed