"query completed without returning a result" panic

[andyleiserson]

Similar to #1334, this panic shows up sometimes as a secondary symptom when a test is failing:

thread 'query::processor::tests::e2e::complete_query_ipa' panicked at ipa-core/src/query/state.rs:116:17:
query completed without returning a result

To avoid possible confusion, it would be better to suppress this and report only the true error. It's also worth making sure the production query flow can't fail this way.