WebDriver "Navigate to URL" returns success on network errors

[SimonSapin]

Steps to reproduce:

• Run ./mach run servo --webdriver 4444
• In another terminal, run PYTHONPATH=tests/wpt/web-platform-tests/tools/webdriver python -c 'import webdriver; webdriver.Session("localhost", 4444).url = "http://this-domain-name-does-not-exist"'

Observed result in 65df9cb: the Servo window shows "Error loading the page", but the Python command runs successfully.

Expected result: as part of executing the command, the WebDriver server inside Servo should run the post-navigation checks, see that the navigation response is a network error, and propagate that error in the WebDriver response. The WebDriver client would then raise a Python exception to indicate this error.

(This came up in #21242, where wptrunner is the WebDriver client.)