scrolling to an error by clicking highlight should have some margins

[dyoo]

Reported by sk:

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If I take the following program, put the cursor somewhere in the middle (so both fragments are off-screen), and run, when I click on the "something else", it only makes the first line visible -- the rest of the purple is still off-screen.

(define (valid-answer? ans

(string=? "yes" (string=? "no"))))

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When we're using the scrollTo method in codemirror, we should provide it a generous margin so that the context is closer to the center of the screen.