DevTools: Fix frontend compilation following r172493

compile_frontend.py bailed out with an exception if a non-existent
file was encountered; no ERROR message was printed. As such,
the compilation succeeded and patch upload was allowed.

R=lushnikov@chromium.org, yurys@chromium.org, lushnikov

Review URL: https://codereview.chromium.org/256543003

git-svn-id: svn://svn.chromium.org/blink/trunk@172499 bbb929c8-8fbe-4397-9dbb-9b2b20218538