Handle GLFW initialization failure

I hit this when trying to run servo in an ssh session. It hit an rtabort with
"fatal runtime error: thread-local pointer is null. bogus!" The exit is
required because at this point, the runtime is in a very weird state, and a
fail is going to explode even more violelently than the original unwrap did.
auto merge of #1502 : cmr/servo/handle_init_error, r=pcwalton

I hit this when trying to run servo in an ssh session. It hit an rtabort with
"fatal runtime error: thread-local pointer is null. bogus!" The exit is
required because at this point, the runtime is in a very weird state, and a
fail is going to explode even more violelently than the original unwrap did.