test_detect_match_nonexistent_template: Cleanup unused import

Thanks to Lewis Haley for spotting this.
test_that_stbt_batch_*: Increase timeouts

These two tests fail intermittently on Travis CI, possibly because we
increased the number of tests that run in parallel in b46b3939.
_numpy_from_sample: Improve error reporting

Print a more useful message when wrong type passed to `_numpy_from_sample`.

I ran into this when mistakenly passing a region as the first argument to
`stbt.ocr`.