Improve errors thrown by client credential provider

[Meierschlumpf]

async authorize(credentials, request) { // you have access to the original request as well
   if(!isValidCredentials(credentials)) {
      throw new CustomError()
   }
   return await getUser(credentials) // assuming it returns a User or null
}

Custom Error here could extend CredentialsSignin error
See: nextauthjs/next-auth#10200