Differentiate suboptimal_program from a program that fails to execute

[ReubenJ]

Currently, when searching using allow_evaluation_errors and all of the programs error out on evaluation (interpret errors out), the return value of synth is marked as a suboptimal_program. This is somewhat confusing because it seems to say that the returned program executed but didn't pass evaluation for some IO examples.

We can either:

• add a third enum value to SynthResult like failing_program
  

  HerbSearch.jl/src/search_procedure.jl

  Line 10 in 5b315ce

  @enum SynthResult optimal_program=1 suboptimal_program=2

• warn that allow_evaluation_errors might mean that the returned suboptimal_program might have thrown an error

[sebdumancic]

This is a good idea!

Especially because if we have a program failing to run, we might be able to deduce additional constraints to remove programs that would fail for the same reason.