Vertical scrollbars in custom message

[davidwagner]

When d8error triggers a custom message, it puts the custom message and the exception backtrace in a fixed-height box on the page. Typically the content is so large that it doesn't fit in the fixed-height box and there's a scrollbar on the right, and some of the content is truncated. This is unfortunate, because we tell people to interpret exceptions by starting at the bottom of the exception message, and that's the part that won't be visible (it'll be "below the fold", and one has to scroll the scrollbar to see it).

It would be better if we could eliminate the fixed-height box and just have the custom message take as long as it takes to display it.