Bug 1399334 - Add more debugging to see why certificates aren't valid. r=ttaubert

There's an intermittent that is showing up now that test_register_sign.html
checks state.attestationCert.verify(); to ensure hte SoftToken's certificate
is valid. This patch prints the offending certificate when it's encountered,
to help diagnose the root cause.

MozReview-Commit-ID: 4QSobq9fBGK