make sure compile-sass exists with non-zero exit code on fail

...and print a message of what went wrong

test plan:

write some bad syntax in a sass file,
run `npm run compile-sass`
it should exit with non-zero status code
it should tell you what line of css caused the error

Change-Id: I0febb3286f6100993d5d2d53083d6a53719cbedf
Reviewed-on: https://gerrit.instructure.com/39385
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>