contenttest: Print stderr
Kill Servo instead of displaying about:failure when running content tests

Fixes #1380.
contenttest: Fail if Servo exits with non-zero status

Remove "let stdout" to make the borrow checker happy.
Don't clear buffers if we have no native graphics context

Fixes ./servo -z for content that calls window.close(), such as content tests.
auto merge of #1382 : kmcallister/servo/headless, r=jdm

r? @jdm