ditch echo for printf & redirect error messages to standard output stream

src/clean-md5-cache

@@ -36,5 +36,5 @@ then
         su root -c "rm -fr ${cache}" || exit 1
     fi
 else
-    echo "No pre-generated md5-cache found in ${cache}"
+    printf "%s\n" "No pre-generated md5-cache found in ${cache}" >> /dev/stderr
 fi