Merge rsync's stdout and stderr instead of reading them sequentially

  - this allows for easier testability

  - it is very useful output

  - eliminates all remaining non-fatal log.Fatal()s
  - situation is now detected when rsync returned without an error
  but still did not create a snapshot

  - if mainExitCode() is != 0 snaprd will send an email to the notify
  address containg the exit code and the last 25 lines of log

  - This enables us to access the last n lines of log output, for
  instance for sending them via email.