[Feature Request] Add errors on sign-in page

[ThePolishLemon]

Please add a few errors and/or visual cues mainly for:

• Wrong username/password
• Unresponsive api
• Potential rate limit (usually these can be fixed by doing a recaptcha on synergia's site)

[SimonB50]

All of the mentioned problems for the sign-in page have been fixed with 9d80aff. Sadly, captcha will require user to log-in through Synergia page, since there is no way of implementing recaptcha in the app.