Segfault occurs if no suitable backend available for an operator

[ms705]

Thanks @n1v0lg for reporting this issue.

When using the heuristic scheduler, we segfault in the scheduler code when there is no suitable back-end for an operator:

I0615 23:11:03.220222 21852 scheduler_dynamic.cc:558] The minimum cost of running the DAG: 100001
I0615 23:11:03.220257 21852 scheduler_dynamic.cc:562] Cur cost: 100001

Program received signal SIGSEGV, Segmentation fault.
0x000000000059c494 in musketeer::scheduling::SchedulerDynamic::ComputeOptimal (this=0x83c800, serial_dag=std::vector of length 3, capacity 4 = {...})
    at /home/malte/Projects/musketeer/src/scheduling/scheduler_dynamic.cc:563
563       uint32_t prev_jobs_exec = parent[cur_cost][cur_jobs_exec];

To reproduce, craft a minimal DAG that contains an operator that cannot be expressed in any of the available execution engines (i.e., for which the framework's scoring method returns FLAGS_max_scheduler_cost), and try to schedule it using the heuristic scheduler.

The solution is two-fold:

1. Fail in a more sensible way.
2. Notice that the DAG is impossible and bail out early, before even invoking the scheduling heuristic.