handle cancel of adding included cards

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We want it to just go back to the gray "Add |cardname|" right? Or at least not show this error message.

On http://test.dwagn.org/wagn/include_a_pointer click to add the card, then cancel. You'll see: "i am an included card - unknown card view: 'content_missing'"

I think this may be specific to cards in content view. regardless, yah, we want to fix this.