test_that_frames_raises_NoVideo: Fix failure on travis-ci

The failure was:

> grep: input file `/tmp/stb-tester.TXa/log' is also the output
Printable error message for NoVideo exception raised by stbt.get_frame

Before this change, the "FAIL: ..." error message printed to stdout by
stbt-run was empty (it just said "FAIL: <test name>: ").