changefeedcccl: move to draining before cancelling proc

Once a proc's context has been cancelled, it may cease processing emitted rows or even
emitted drain transition information, so if the processor implementation wishes to
reliably emit information about why it is moving to draining, it should do so before
it cancels its underlying base processor.

Release note: none.
Epic: none.

Author: dt

pkg/ccl/changefeedccl/changefeed_processors.go

@@ -814,10 +814,10 @@ func (ca *changeAggregator) Next() (rowenc.EncDatumRow, *execinfrapb.ProducerMet
 					err = kvFeedErr
 				}
 			}
-			// Shut down the poller if it wasn't already.
-			ca.cancel()
 			log.Changefeed.Warningf(ca.Ctx(), "moving to draining due to error from tick: %v", err)
 			ca.MoveToDraining(err)
+			// Shut down the poller if it wasn't already.
+			ca.cancel()
 			break
 		}
 	}