Doorkeeper can be misleading when certain errors occur in custom token generators

[MarkMurphy]

See my comment in the NoMethodError handler

# `access_token_mixin.rb`
  def generate_token
      self.created_at ||= Time.now.utc

      generator = Doorkeeper.configuration.access_token_generator.constantize
      self.token = generator.generate(
        resource_owner_id: resource_owner_id,
        scopes: scopes,
        application: application,
        expires_in: expires_in,
        created_at: created_at
      )
    rescue NoMethodError
      # Doorkeeper::JWT was trying to use `[]` on a null variable and thus a NoMethodError was raised. The error message returned here in this case was misleading because Doorkeeper::JWT does in fact respond to `.generate`
      raise Errors::UnableToGenerateToken, "#{generator} does not respond to `.generate`."
    rescue NameError
      raise Errors::TokenGeneratorNotFound, "#{generator} not found"
    end

[anthonylebrun]

I believe I had just had this happen to me. I think the solution may be to use a throw a custom exception rather than use NoMethodError and explicitly check via respond_to?(:generate). Happy to do a PR if I get the green light.

[Charizard]

Same here 👍