Better error reporting & exception handling

[edgarRd]

Need to revise and improve error reporting in callbacks as well as exception handling in all Stardog.js functions.

Error reporting in callbacks should follow the common JS style as:

function cb (err, result)

[jwulf]

I'd especially like some form of diagnostic reporting for Connection.query. At the moment it either works, or doesn't.

[schwarzmx]

Fixed in this commit.