go() should error when no url

[marutypes]

It is extremely difficult to debug core-ajax elements with bindings due to the fact that they simply d not fire when something is wrong with the url attribute. I propose that go() check for a truthy url and only proceed if it finds one.

Also, I realize that currently the function returns false when the go does not successfully activate the query. I think throwing an error would be more developer-friendly however. It would make problems obvious in the console and can be equivalent to the return value model using try/catch blocks.