Add exception for handling invalid request token

[wallin]

Introduce a separate exception InvalidRequestTokenError for handling missing or invalid request token. Per default, Castle recommends rejecting these requests, as opposed to other 422 status errors, which are indicators of integration errors.

Example usage:

try {
  castle.risk(...)
} catch (e) {
  if (e instanceof InvalidRequestTokenError) {
     // Block request
  } else if (e instanceof InvalidParametersError) {
     // Data missing or invalid. Needs to be fixed
  }
}