dataset: code hygiene for rsync-dumps.py

* drops unused imports (os, re)
* span list comprehensions on several lines, easier to read imho
* drop exception definition which was never used
* Rsyncer.do_rsync() had an undefined variable when calling
  communicate(), might have caused some issue when sending mail
  notifications.
* split overlong comment lines

Change-Id: Ifdb0a680ea2b692c5f99123f50e7737efc08e689