talksapp: remove decoration to errors at top level

The errors returned are used to decide what status code
should be set on the http response.

Fixes #441.

Change-Id: Ia01ed2186dd6d23cd38402888df7d2de3f687d0a
Reviewed-on: https://go-review.googlesource.com/28781
Reviewed-by: Chris Broadfoot <cbro@golang.org>