Don't fail on missing images when resizing

The changed code obtains the image dimensions when you first upload an
image. Unfortunately it's also triggered when thumbnails are refreshed.
That doesn't change the size of the original image though.

app/models/spree/image.rb

@@ -34,6 +34,8 @@ module Spree
     end
 
     def find_dimensions
+      return if attachment.errors.present?
+
       temporary = attachment.queued_for_write[:original]
       filename = temporary.path unless temporary.nil?
       filename = attachment.path if filename.blank?