Added --quiet (no warning on missing files)

This lets me take advantage of my new 'sublx' script's ability
to open project files easily.

Where the editor was set to 'subl', opening a project involved this:
    cedit -- --project /home/me/projects/myproject.sublime-project

But since my 'sublx' script allows this (for existing projects):
    sublx myproject

I can now do this:
    cedit myproject -q

..Which is much better than the original :)