properly deal with broken statefiles, and avoid them

[Dieterbe]

I ran out of diskspace and apparently it stored a broken statefile, which breaks subsequent runs.

root        : INFO     Rss2email started
Traceback (most recent call last):
  File "/usr/bin/rss2email.py", line 883, in <module>
    run()
  File "/usr/bin/rss2email.py", line 605, in run
    feeds = load()
  File "/usr/bin/rss2email.py", line 565, in load
    feeds_state = pickle.load(statefile)
EOFError

So,

• handle the case of broken statefiles better (abort with proper error)
• avoid this from happening (create tmp file first, then move it)