add recovery in top level handler

[jaffee]

Overview

Previously, if Pilosa experienced a panic while handling a request, it would print the stack trace, abort handling of the current request, and continue handling other requests as normal - the client was left in the lurch with an unexpected EOF. This PR recovers from all panics in handlers explicitly and returns an internal server error to the client along with a stack trace so the client may handle the failure more gracefully.

Fixes #

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[alanbernstein]

Is there excessive overhead from this import, or some other drawback?