post-review tool fails if file is not readable (or other diff issue)

Review Request #1457 — Created March 5, 2010 and submitted — Latest diff uploaded

If diff file is not empty, post-review wrongly assumes that the second line
exists.  This causes the script to abort because of an exception on dl[1]
as the index is out of range.

Verified after marking a file as not readable.
Handling the failure has also the nice benefit that temporary files are properly cleaned up.

ERROR, no valid diffs: diff: <some file name>: Permission denied