DevTools: Fix compile_frontend.py error reporting for frontend_modules.json

R=lushnikov
BUG=403309
NOTRY=true

Review URL: https://codereview.chromium.org/466393002

git-svn-id: svn://svn.chromium.org/blink/trunk@180170 bbb929c8-8fbe-4397-9dbb-9b2b20218538