Print GLFW errors
Enable DOMParser test case

Now that #1071 is fixed.
Remove special-casing of URLs ending in ".js"

This was a very old (May 2012) testing feature which used
std::io::read_whole_file rather than our normal resource-loader mechanism.

We can implement javascript: URLs later.
Factor out a convenience function load_whole_resource

Also remove an unnecessary spawn in js_script_listener, and remember the final
script URL after redirects.
Add a spawn_with! macro and clean up some spawns