guard: print handlebars errors

fixes CNVS-11640

test plan:
  * start guard
  * introduce a syntax error in a handlebars file
  * verify that the error is displayed

Change-Id: I17f624a709c96dfbd9114b4c7204c8bec51cd425
Reviewed-on: https://gerrit.instructure.com/31340
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>