Add error message for readlink -f failure (#23102)

* Add error message for readlink -f failure

scripts/cargo-for-all-lock-files.sh

@@ -3,6 +3,13 @@
 here="$(dirname "$0")"
 cargo="$(readlink -f "${here}/../cargo")"
 
+if [[ -z $cargo ]]; then
+  >&2 echo "Failed to find cargo. Mac readlink doesn't support -f. Consider switching
+  to gnu readlink with 'brew install coreutils' and then symlink greadlink as
+  /usr/local/bin/readlink."
+  exit 1
+fi
+
 set -e
 
 shifted_args=()