Error out when no key in JGET

Right now no-key error only happens in jget*, but it's intended to happen in the regular jget starting from the next major release. The benefit is: malformed JSON is easier to spot and fix, while jbind patterns are behaving in a more correct way in regards to validation.

Yes, panicking more often might be annoying, but this library is intended to be interactive, so it's better to error than not—to be honest to the user.