Better signin/signup error messages!

* Allow users who sign up with an existing account to sign in. If they type an  incorrect password, show them a link to the /signin page.
* Show users who sign in with a non-existent account a link to the /signup page.

* Clarify & simplify error processing of errors returned by the auth server
* Add a table of human understandable string types->numeric codes returned by the auth server to auth-errors.js
* All access to auth-errors goes through three functions, toMessage, toCode, and is;
* Add displayErrorUnsafe - no conversion on HTML before being displayed.
* Add functional tests for /signin and /signup error cases.
Merge pull request #542 from mozilla/issue-182-sign-in-messages

 Better signin/signup error messages!