Bubble up image download failures

Currently, we're still returning 0 out of the upload_image method
despite the download failing. This changes behavior such that if the
image download fails, it returns an exit code of 1 to the caller to be
handled (or fail early) accordingly.

Change-Id: I901dc065b51946f363145ae888cca602946ceeea

functions

@@ -147,7 +147,8 @@ function upload_image {
                 if [[ $rc -ne 0 ]]; then
                     if [[ "$attempt" -eq "$max_attempts" ]]; then
                         echo "Not found: $image_url"
-                        return
+                        # Signal failure to download to the caller, so they can fail early
+                        return 1
                     fi
                     echo "Download failed, retrying in $attempt second, attempt: $attempt"
                     sleep $attempt