Sort and uniqify dangling rules list

Make the dangling rules list sorted and unique in order to avoid
very long lists when a dangling rule is referenced many times.

Also prettify the output by indenting the list and printing
"stopping" instead of a blank line for the fatal.

Test: m checkbuild
Change-Id: I8f7c27ae39b59f506b529d9995d90b0d6b9835d1