Python runner should throw an exception when action result can't be serialized

[Kami]

Right now if the result can't be serialized we return str() version of the result, but I believe the right approach would be to throw an exception in such scenario.

str() version of the result is usually of little use and not being able to de-serialize the result usually indicates a programmer error (action / pack developer didn't correctly implement de-serialization of the result in format which can be consumed in StackStorm).

If people strongly believe there still is some value in str() version of the result we can add allow_non_serializable_results Python runner config option which should default to false.

For details, see: #3489, #3540.

[nmaludy]

+1