module doesn't create "issue directory" automatically

whatever subdirectory you select via [site]/admin/settings/project_issue as the "Issue directory" is not validated and created for you if it doesn't exist. this results in weird behavior if you forget to manually mkdir yourself (all issues get uploaded as "issue" and you start hitting the file renaming stuff instead of using the names of the files you're trying to attach). plus, i guess some sites don't have shell access, where creating this directory would be more of a pain in the ass. we should validate to make sure the name they choose is a writeable directory or doesn't exist at all. if it's not there, we should try to create it, and mark the field as an error if we fail.