change error message of "Variable is undefined" to actually show the offending name. This makes variable stream consistent with the variable op.
change error message of "Variable is undefined" to actually show the offending name. This makes variable stream consistent with the variable op.

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Imported from Google Code.
Original SVN rev#: 141

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