Capture panics in HTTP handlers, and add a critical log line with the panic information.
Capture panics in HTTP handlers, and add a critical log line with the panic information.

Change-Id: I46e236c84be3854d470b41f8110b010eed5f562a

skyeunknown084/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,mzimmerman/appengine,golang/appengine,flowlo/appengine,flowlo/appengine,mzimmerman/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,flowlo/appengine,skyeunknown084/appengine,mzimmerman/appengine,golang/appengine,skyeunknown084/appengine