Bug 1027740 - Make accessibility UI test failures more verbose

This is the intended result. We catch any exception in the callback and report it back to python and throw it there. In the case of getName or getRole, we will get an exception message that acc is null, which will make debugging easier (yes we could null-check in those callbacks but the default exception message is useful enough already).

Also, !acc is not necessarily an error, it may be expected.