added an error handler for zipkin-web
This commit adds some slighly hackish error handling code that should exist in Finatra.
I submitted a patch to finatra, and it was accepted, and julio rolled a release, but it depends on finagle 6
which breaks our dependency tree.  Sooo I'm forced to put this code in here with a FIXME because I want
the feature.
Merge pull request #233 from twitter/web-error-status

Error handling for zipkin-web.  Before we were returning an empty document if a downstream exception happened, now we print the stacktrace to the screen.  There is some hackish code in here that is needed until we upgrade to Finatra 1.3.0 + Finagle 6 + Cassie 0.?? (cassie not released yet)