Fixing several (but not all) go vet errors. Most are around string formatting, or unreachable code.

pkg/client/testing/core/fixture.go

@@ -97,8 +97,6 @@ func ObjectReaction(o ObjectRetriever, mapper meta.RESTMapper) ReactionFunc {
 		default:
 			return false, nil, fmt.Errorf("no reaction implemented for %s", action)
 		}
-
-		return true, nil, nil
 	}
 }