Hide the error output from sed(1) unless --verbose is used

It's unlikely that sed(1) will generate any error output, but still
it's good to handle it for the sake of completeness.

fedora-toolbox

@@ -289,7 +289,7 @@ while [[ "$1" = -* ]]; do
                 echo "Try '$0 --help' for more information."
                 exit 1
             fi
-            arg=$(echo $1 | sed 's/^F\|^f//')
+            arg=$(echo $1 | sed 's/^F\|^f//' 2>&42)
             if ! is_integer $arg; then
                 echo "$0: invalid argument for '--release'"
                 echo "Try '$0 --help' for more information."