wysiwyg_get_css should be checking whether css files exist

wysiwyg_get_css() might cause editors to try including css files that don't exist. The attached patch causes it to check whether a css file exists before it returns it with the rest of them.

Drupal's drupal_get_css() does it (calls file_exists() on each css file), and it looks like wysiwyg_get_css() is modelled after drupal_get_css(). Both use the output from drupal_add_css() to determine what css files should be included. The major differences seem to be that wysiwyg_get_css() only looks at theme css, and it returns all the files as an array, instead of html link tags.

Before I applied this patch to our installation, we've been getting random page not found errors, and I traced it back to that function. It is normal for a theme to include nonexistent css files to prevent other ones with the same name from loading:
http://drupal.org/node/263967