DevTools: Add error handling to closure runner

The runner would previously hang if an invalid command line would get passed to it.

Drive-by: Explicitly use the UTF-8 charset for the input file.

R=eustas, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/149473002

git-svn-id: svn://svn.chromium.org/blink/trunk@166129 bbb929c8-8fbe-4397-9dbb-9b2b20218538