Fixing potential key errors on retrieving distribution

Tactical fix for distribution error stack trace.

I see the following quite a few times.

ERROR: Unhandled exception; shutting down
Traceback (most recent call last):
  File "/usr/bin/mini-dinstall", line 531, in run
    self._daemonize(initial_reprocess_queue, initial_fucked_list)
  File "/usr/bin/mini-dinstall", line 672, in _daemonize
    self._reject_changefile(changefilename, changefile, DinstallException("Couldn't install \"%s\" in %d seconds" % (changefilename, self._max_retry_time)))
  File "/usr/bin/mini-dinstall", line 597, in _reject_changefile
    dist = changefile['distribution']
  File "/usr/lib/python2.7/UserDict.py", line 23, in __getitem__
    raise KeyError(key)
KeyError: 'distribution'