Add meaningful exception when the device is in an unknown state

Currently, when the device is in an incorrect state (e.g. in split
screen) because another test failed to teardown the error message is a
java.lang.NullPointerException. Instead of this error, show a message
warning about the problem to aid debugging.

Test: atest FlickerTests
Change-Id: If3bd75c14c90209952484802ee17f817ab4f0f20