deploy-order: fail if non-existing app is specified

If an application was specified to calculate the deploy-order for that
did not exist in the tree, the app was added to the tree as
undeployable, eventually included in the deploy order and the command
succeed.

Fail instead if the app does not exist in the dependency tree.
This will allow to detect during deployment if an app was specified that
does not exist in the exported dependency tree

internal/deps/composition.go

@@ -192,10 +192,9 @@ func (comp Composition) RecursiveDepsOf(s string) (newcomp *Composition, err err
 
 	for len(todo) > 0 {
 		for serviceName := range todo {
-			service, ok := comp.Services[serviceName]
-
+			_, ok := comp.Services[serviceName]
 			if !ok {
-				comp.AddService(serviceName, NewService(service.Deployable))
+				return nil, fmt.Errorf("application %s does not exist", serviceName)
 			}
 
 			newcomp.Services[serviceName] = comp.Services[serviceName]

internal/deps/composition_test.go

@@ -85,17 +85,6 @@ func TestRecursiveDepsOf(t *testing.T) {
 	}
 }
 
-func TestRecursiveDepsOfWithListOfServices(t *testing.T) {
-	comp := newTestComp()
-
-	got, _ := comp.RecursiveDepsOf("fourth-service,first-service")
-
-	_, ok := got.Services["fifth-service"]
-	if !ok {
-		t.Error("expected to have 'fifth-service' in composition")
-	}
-}
-
 func TestRecursiveDepsOfWithListOfServicesAndBlank(t *testing.T) {
 	comp := newTestComp()
 	got, _ := comp.RecursiveDepsOf("fifth-service, fourth-service")
@@ -215,3 +204,12 @@ func TestDeployOrderWithUndeployables(t *testing.T) {
 	cmpSlice(t, []string{"d"}, order)
 
 }
+
+func TestRecursiveDepsOfFailsIfUnknownAppIsSpecified(t *testing.T) {
+	comp := NewComposition()
+	_, err := comp.RecursiveDepsOf("xyz")
+	expectedErrStr := "application xyz does not exist"
+	if err == nil || err.Error() != expectedErrStr {
+		t.Fatalf("expected err with msg: %q, got: %v", expectedErrStr, err)
+	}
+}