Log more information to the console.

This is principally needed to debug issues on the Chromium project's automated test machines.
Added an error catch to all instances of canvas.toDataUrl, to properly report intermittent failures seen in Chrome.
Merge pull request #679 from kenrussell/log-debug-to-console

Log more information to the console.
Addressed freedback from Ken
Merge pull request #680 from toji/to_data_url_error

Added an error catch to all instances of canvas.toDataUrl
Forgot to add "FAIL" to failing messages logged to console.
Merge pull request #681 from kenrussell/fail-message

Forgot to add "FAIL" to failing messages logged to console.