"execute" endpoint returns SMT error instead of `Branching` result

[geo2a]

Consider the following scenario:

• Proxy receives an "execute" request
• Booster aborts and falls back to Kore
• Kore's "execute" handler cannot decide a rule side-condition, emits a DecidePredicateUnknown warning and returns Branching
• Proxy gets the branching result and applies the post-exec simplification to the three sub-states: the pre-branch state and two post-branch states. Now, the post-exec simplification effectively sends and internal "simplify" request to Kore. Naturally, it fails to decide the same predicate (that's Kore branched in the first place), but now it throws an DecidePredicateUnknown error
• The error is caught by the Proxy's handler, wrapped into JsonRPC error and returned to the client.

In the end, the client gets a not-so-helpful error (which we've just made a little more helpful in #371). What the client should get instead is the Branching response, and, perhaps, the predicate that could not be decided.

I suppose the same problem is the case for all other execute results as well.

[geo2a]

The culprit seems to be Kore.Rewrite.SMT.Evaluator.decidePredicate, which throws ErrorDecidePredictateUnknwon when called by Kore.Rewrite.SMT.Evaluator.filterMultiOr.

Do we ever want decidePredicate throw DecidePredictateUnknwon as error, rather than emitting a warning? We probably do not want the error when we process an "execute" request. However, when processing "simplify", we may want the exception, so that we can catch it and return to the client the unknown predicate to pretty print.