Provide helpful error if token isn't a valid JWT

[nathan-collins]

Auth store

When I assign a token to the auth store it throws an error when it attempts to decode the token. (HTMLElement._tokenChanged (auth-store.html:139)) the token is present at this point.

Uncaught TypeError: Cannot read property 'replace' of undefined at b.exports (jwt-decode.min.js:1) at b.exports (jwt-decode.min.js:1) at HTMLElement._tokenChanged (auth-store.html:139) at HTMLElement._observerEffect (polymer.html:1662) at HTMLElement._effectEffects (polymer.html:1507) at HTMLElement._propertySetter (polymer.html:1491) at HTMLElement.setter (polymer.html:1570) at HTMLElement._formResponse (email-login-form.js:71) at HTMLElement.handler (polymer.html:566) at HTMLFormElement.fire (polymer.html:1343)