Apply resource pattern to error describer service

- Follows Resource pattern (IdentityErrorDescriber.StringName, or FormatStringName(arg1, arg2)
- Also cleaned up optional services, by allowing null in constructor
- SignInStatus -> SignInFailure and introduced SignInResult to make SignInManager APIs consistent with IdentityResult (but no strings needed for SignIn)

Fixes: https://github.com/aspnet/Identity/issues/86, https://github.com/aspnet/Identity/issues/176, https://github.com/aspnet/Identity/issues/287 and https://github.com/aspnet/Identity/issues/177

cc @divega @suhasj 

For this particular case, we could just make this a public get/set auto property too, it doesn't really matter much here since its just the validator