Throw an error if the user attempts to call $get_image with a key that hasn't been loaded.
Throw an error if the user attempts to call $get_image with a key that hasn't been loaded.

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Imported from Google Code.
Original SVN rev#: 135

blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon