Add custom error types when handling errors

[jbmusso]

Consider passing custom/explicit error types to the callback when errors occur.

I can think of these errors right now:

• NetworkError Ex: no internet connection, should be easy to catch at the http module level
• ServerError Ex: Rexster returns with a 500 error for some reasons (the HTTP code should be explicitely returned as well).
• TimeoutError Ex: connected, but script timed out for various reasons (connection timeout? script timeout?)
• ScriptError Ex: exception thrown by Rexster when executing the script.

There could be more (or less) errors. Maybe ConnectionError and ClientError. Thoughts welcomed!