Buildd-manager fails horribly if the uploader.log is not found after a build upload

In the Medusa-like code that lives in lib/lp/buildmaster/buildergroup.py / buildStatus_OK() it attempts to deal with the Popen of process-upload failing.

However, if the process fails in a way where it has no chance to write uploader.log in the relevant directory, the code at the end of that method fails horribly as it blindly tries to open the log file, because it only checks to see if the directory it lives in exists.

This in turn causes a scan failure in the buildd-manager which completely blocks any more builds from being dispatched. It should fail gracefully instead and write something about not finding the upload log. In fact ideally this Popen crack should be removed entirely and a call to a script hook should be done instead, but that can wait.