realtime_zsl: Make a helper to combine error and pending entries

Moves the logic to a helper function that combines error and pending
entries into the result.

This commit doesn't change functionality, and is meant to allow future
commits to re-use the same logic.

Bug: 235371520
Test: Framework stress test, CTS
Change-Id: I6a751794b4f5d087280708278f13769fe18cdb34