getDiskStats should be more robust to df failures

[adamgundry]

My local df command emits an error df: /run/user/1000/doc: Operation not permitted and returns an exit code of 1. (I'm not sure why.) It still produces useful output on stdout.

Since getDiskStats in Ridley uses readProcess, it will pass through stderr to the parent process, and will throw an exception when df returns a non-zero exit code. Instead, using readProcessWithExitCode would allow the exit code to be logged but not treated as a complete failure. Moreover the stderr output could be logged normally rather than being passed through to the parent process.