Catching response errors mechanism was rewritten

There are several errors in catching errors mechanism in transmit function

1. $this->xml is boolean and can't be an instance of SimpleXMLElement
2. When there are some errors with authentication, QuickBase sends xml response with:
Sorry!  Something's wrong with your sign-in info. Try again.

That's why authentication error message was removed.
3. There is no any check for response if it isn't valid SimpleXMLElement. For example $r could contain string: "Bad request". In this case creating of SimpleXMLElement object will crash
4. Warnings for incorrect creation of SimpleXMLElement were removed
5. Response text was added to error message