Correctly handle error response codes when a target disappears

[pmeenan]

This fixes an unnecessary delay when trying to retrieve response bodies from a target that has disappeared. Chrome returns an error to the outer dev tools command but we previously always expected it to succeed so it has to time out the command.

This should speed up testing a bit (encountered it while testing some wappalyzer updates).

[tkadlec]

Thanks @pmeenan! Had done some profiling of the agent and Wappalyzer was one of, if not the, most expensive parts. Was looking at making it optional, but I'll run a few tests and see how much faster this made things first.

[pmeenan]

This fix should have been independent and won't make wappalyzer faster, sadly. It is mostly around getting response bodies where it seemed to trigger the error. Making wappalyzer optional makes 100% sense.

[tkadlec]

Ah, gotcha. Sorry...misread the description.