[ResultDB] Improve resultsink logging.

Log the test process error, if any.

BUG=None

Change-Id: Icb90d52131d7204166a32f9f6823b4abcb1f9840
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/6014018
Auto-Submit: Patrick Meiring <meiring@google.com>
Commit-Queue: Beining Chen <beining@google.com>
Reviewed-by: Beining Chen <beining@google.com>
diff --git a/resultdb/cli/cmd_stream.go b/resultdb/cli/cmd_stream.go
index 45453d5..36b761a 100644
--- a/resultdb/cli/cmd_stream.go
+++ b/resultdb/cli/cmd_stream.go
@@ -496,9 +496,15 @@
 		cmdProcMu.Unlock()
 
 		if err != nil {
+			logging.Warningf(ctx, "rdb-stream: failed to start test process: %s", err)
 			return errors.Annotate(err, "cmd.start").Err()
 		}
-		return cmd.Wait()
+		err = cmd.Wait()
+		if err != nil {
+			logging.Warningf(ctx, "rdb-stream: test process exited with error: %s", err)
+			return err
+		}
+		return nil
 	})
 }