Refactor: Decouple failed attachments and the flash messages

Attachment#attach_files will no longer need to return a flash message,
instead it will put unsaved attachments into object#unsaved_attachments
where the calling object can access them.

A utility method #render_attachment_warning_if_needed is included for setting
the standard flash warning.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3528 e93f8b46-1217-0410-a6f0-8f06a7374b81