veyron/{services/mgmt,tools}/build: fixing the build server shell test

Change-Id: Icfbab43b9e6893588d91bad117c862cc8e549353
diff --git a/tools/build/impl/impl.go b/tools/build/impl/impl.go
index f575475..eb76237 100644
--- a/tools/build/impl/impl.go
+++ b/tools/build/impl/impl.go
@@ -178,7 +178,7 @@
 			return
 		}
 		if out, err := stream.Finish(); err != nil {
-			errchan <- fmt.Errorf("Finish() failed: (%v, %v)", out, err)
+			errchan <- fmt.Errorf("Finish() failed: (%v, %v)", string(out), err)
 			return
 		}
 		errchan <- nil