jqueue: Mark opcodes following failed ones as failed, too

When an opcode fails, the job queue would leave following opcodes as “queued”,
which can be quite confusing. With this patch, they're all marked as failed and
assertions are added to check this.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>