arm_trusted_firmware_uprev: Clarify error message for most common case

Direct Git push access must always be restricted, so the
arm_trusted_firmware_uprev script only works for people who're part of
the chromeos-arm-trusted-firmware-pushers Ganpati group. This is easy to
forget when telling new developers who just started to work on Arm TF
about the script. This patch adds a more explicit error message to this
case to help point them in the right direction.

BUG=none
TEST=none

Change-Id: I2c1daeb0ef281f7d666f09069dd1561f2a17e311
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1103387
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
diff --git a/arm_trusted_firmware_uprev b/arm_trusted_firmware_uprev
index 50aa30e..d902a02 100755
--- a/arm_trusted_firmware_uprev
+++ b/arm_trusted_firmware_uprev
@@ -134,7 +134,10 @@
       "${PREV_REMOTE}2":"${PREV}3" \
       "${PREV_REMOTE}1":"${PREV}2" \
       "${MASTER_REMOTE}":"${PREV}1" \
-      "${commit}":"${MASTER}" || die_notrace "Push failed (see above)"
+      "${commit}":"${MASTER}" || die_notrace "Push failed (see above).
+If you are seeing authorization errors ('update access denied'), you are
+probably not a member of the chromeos-arm-trusted-firmware-pushers Ganpati
+group. You can reach out to any current member of the group to add you."
   fi
 
   echo "Push successful!"