Allow middleman sass to not throw error when images are missing in assets
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I am using Middleman (a sinatra app for front-end demo development). It has sass and asset pipeline. Currently I have this sass file, and many rules references background images:

      a
        background: image-url("main_layout/blog.png") no-repeat left top

However a lot of images are missing, and when this happens, Middleman will give me this error page:

    Error compiling CSS asset
    Errno:ENOENT: No such file or directory - (path)

Is it possible to configure Middleman (or sass) so that a missing image reference in sass won't cause error when viewing a page. I want this because these sass files are used in Rails and won't throw exception on the page.