Better errors from build-flag

When `get_build_var` fails, output the error.

Bug: none
Test: manual
Change-Id: If4dc13e0e895823f50de1f5c7e929bb05eac69a0

cmd/release_config/release_config_lib/util.go

@@ -216,6 +216,7 @@ func GetDefaultMapPaths(queryMaps bool) (defaultMapPaths StringList, err error)
 		var stdout strings.Builder
 		getBuildVar.Stdin = strings.NewReader("")
 		getBuildVar.Stdout = &stdout
+		getBuildVar.Stderr = os.Stderr
 		err = getBuildVar.Run()
 		if err != nil {
 			return