Bug 966106 - Part 1: style links in error box.
Bug 966106 - Part 2: refactor showClientRemoteException.
Bug 966106 - Part 3: refactor doSigninInstead.
Bug 966106 - Part 4: rename signInInsteadLink to createAccountInsteadLink.
Bug 966106 - Part 5: show clickable 'Sign in' link if an account already exists during creation.
Bug 966106 - Review comment: tweak doSigninInstead.
Bug 966106 - Detect existing accounts during account creation, and offer to sign in. r=nalexander