When we expand a number range, we just print "$id - unknown test,
ignored", this is convenient if we want to run a range of tests.

When we designate a test case number explicitly, we shouldn't just
ignore it if the case script doesn't exist.

Print an error and fail the test.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>