Better error messages

[My1]

from #191
https://developers.yubico.com/U2F/Libraries/Client_error_codes.html

The error codes are the following:
1: Something not listed below (aka unknown error)
2: Bad Request, aka:

• the AppID is invalid e.g.
  • no HTTPS,
  • URL Doesnt match the site you are on
  • U2F is called wrongly

3: Something wrong with the Client config (no Idea what that means)
4:
in registration: U2F stick already registered
in Login: U2F Stick unknown for this account

5: Timeout

[gstammw]

There's no error message being shown when one inserts the wrong Securitykey, this should be fixed.

[My1]

well when inserting the key the site doesnt even know what happens, but after pressing it, sure it should show an error 4

[iandunn]

U2F is deprecated and no longer works in Chrome, so the provider is being removed in #439.

A WebAuthn provider is being added in #427 , though, and I think this'll probably apply there too.

[kasparsd]

Closing since U2F is being removed in #439. The webauthn implementation will require that we handle this properly!