code: static Object getInMessage(PMessage message, String key) throws ProvidenceConfigException {
          return getInMessage(message, key, null);
      }
docs: Look up a key in the message structure. If the key is not found, return null.
  
  @param message The message to look up into.
  @param key The key to look up.
  @return The value found or null.
  @throws ProvidenceConfigException When unable to get value from message.