UnconfiguredDeviceUnderTest.draw_text: Don't raise exception
UnconfiguredDeviceUnderTest.draw_text: Don't raise exception

You should be able to call `draw_text` when you don't have a real device
under test (e.g. during test-development when you're trying out
`match()` against saved screenshots).

Really this shouldn't belong in the DeviceUnderTest at all, but that
would require pulling `SinkPipeline` out of `DeviceUnderTest`, which is
a larger refactoring that I'm not prepared to do right now.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester