DevTools: Improve error reporting for compile_frontend.py

R=pfeldman, vsevik, yurys
NOTRY=true

Review URL: https://codereview.chromium.org/148343004

git-svn-id: svn://svn.chromium.org/blink/trunk@166707 bbb929c8-8fbe-4397-9dbb-9b2b20218538