Set proper exit status when Merge Trains fail

When triggering a merge train for stable branches, Curl would exit with
a status of 0 in the event of a failure (e.g. a wrong token being used).
By using the -f flag, we tell Curl to exit with a non-zero exit code
instead. This in turn will result in build failures, making it more
obvious to the Delivery team that something is wrong.

scripts/sync-stable-branch.sh

@@ -51,7 +51,8 @@ then
   echo 'Commit not available, triggering a merge train'
 fi
 
-curl -X POST \
+curl -f \
+  -X POST \
   -F token="$MERGE_TRAIN_TRIGGER_TOKEN" \
   -F ref=master \
   -F "variables[MERGE_FOSS]=1" \