embed: Make failure to start web extension connection fatal

If we can't start the web extension D-Bus server, tons of Epiphany features will be broken. This is unacceptable. It shouldn't be a harmless warning, it should be fatal.

Similarly, if a web process cannot connect to the web extension server, it should immediately crash so that we can debug the problem rather than suffer silent failures.

This might help with debugging #871 (closed), and would at least rule out this as a possible cause of failure. But it's prompted by #887 (closed), where we see this actually happening to a user in practice.

Not that further errors from the web extension are intentionally not made fatal, because a lot could conceivably go wrong, and fatal is very harsh. But the initial step of establishing a connection, at least, should never be allowed to fail.