Auto merge of #4806 - alexcrichton:fix-infinite-loop, r=matklad

Fix an infinite loop in error reporting

The `path_to_root` function unfortunately didn't account for cycles in the
dependency graph introduced through dev-dependencies, so if a cycle was present
then the function would infinitely loop pushing items onto a vector.

This commit fixes the infinite loop and also touches up the reporting to be a
little more consistent with the rest of Cargo