s3_upload: improve url validation and error message

Ensure s3_upload correctly validates the S3 folder url by requiring it
to end with /. This prevents wrong uploads to invalid paths, such as
file urls. Also improve the error message.

Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34255>

.gitlab-ci/piglit/piglit-traces.sh

@@ -127,7 +127,7 @@ replay_s3_upload_images() {
             __DESTINATION_FILE_PATH="$__S3_TRACES_PREFIX/${line##*-}"
         fi
 
-        s3_upload "$RESULTS_DIR/$__PREFIX/$line" "https://${__S3_PATH}/${__DESTINATION_FILE_PATH%/*}"
+        s3_upload "$RESULTS_DIR/$__PREFIX/$line" "https://${__S3_PATH}/${__DESTINATION_FILE_PATH%/*}/"
     done
 }