Fix incorrect outcome check for polling in update_with_start

[GregoryTravis]

Fix execute_update_with_start_workflow raising RPCError NOT_FOUND on validator rejection

Fixes #454.

Bug

When a workflow_update_validator rejects an update sent via
execute_update_with_start_workflow, the caller received
Temporalio::Error::RPCError with code NOT_FOUND instead of
Temporalio::Error::WorkflowUpdateFailedError carrying the validator's
exception as cause.

Root cause

The outcome-present check sense was wrong, so it did a history poll for the outcome that it already had and would not get from history, resulting in an RPCError..

Test: test_update_with_start_validator_rejection

[chris-olszewski]

We should add this fix to the changelog