Output error message when FileReader for a blob fails
Output error message when FileReader for a blob fails

R=tyoshino@chromium.org

Review URL: https://codereview.appspot.com/141010043
google/pywebsocket,googlearchive/pywebsocket,GoogleChromeLabs/pywebsocket3,GoogleChromeLabs/pywebsocket3,GoogleChromeLabs/pywebsocket3,googlearchive/pywebsocket,googlearchive/pywebsocket,google/pywebsocket,google/pywebsocket