Wrap the context render panic.

[danmux]

The utility of the c.JSON helper or other context.Render methods not returning an error simplifies client code.

There are rare errors during rendering that result in a panic, which most production systems will handle in middleware.

This introduces the ability to detect that it is a gin rendering error in the panic handler as opposed to other handler logic panics.

To be able to trap c.JSON render panics this PR will also need #3390 - which changes the JSON renderer to be more consistent, until that is also merged this context Render panic will not fire.

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[danmux]

better handled by. https://github.com/gin-gonic/gin/pull/2150/files