whoopsie.postinst crashes silently if /var/crash is missing

In a fresh install, such as when building an image - e.g. http://people.canonical.com/~ubuntu-archive/livefs-build-logs/precise/ubuntu-zh_CN/20120410/livecd-20120410-i386.out - whoopsie may be configured before apport or (hopefully!) before anything crashes, and in that case it will fail to configure because /var/crash does not exist yet. Worse, it does so silently, with the only clue being:

  Setting up whoopsie (0.1.27) ...
  dpkg: error processing whoopsie (--configure):
   subprocess installed post-installation script returned error exit status 1

1) whoopsie.postinst should run 'mkdir -p -m 3777 /var/crash' before operating on it.

2) '/dev/null 2>&1' should be removed from the first pair of chmod/chgrp calls, the ones that may cause the postinst to fail. I can understand why you wouldn't want error output from the second pair, since you'll get glob failures if there are no *.crash files in /var/crash/, but if either 'chmod g+s /var/crash' or 'chgrp whoopsie /var/crash' emits any output then it will almost certainly be a reason for exiting non-zero and such error output should not be thrown away.