give warning when trying to convert child of prefab asset

Author: vkovec

com.unity.formats.fbx/Editor/ConvertToNestedPrefab.cs

@@ -161,6 +161,13 @@ public static GameObject[] CreateInstantiatedModelPrefab(
                         return null;
                     }
 
+                    if (PrefabUtility.IsPartOfPrefabAsset(go) && go.transform.parent != null)
+                    {
+                        DisplayInvalidSelectionDialog(go,
+                            "Children of a Prefab Asset cannot be converted.\nYou can open the Prefab in Prefab Mode or unpack the Prefab instance to convert it's children");
+                        return null;
+                    }
+
                     // can't currently handle converting root of prefab in prefab preview scene
                     if (SceneManagement.EditorSceneManager.IsPreviewSceneObject(go) && go.transform.parent == null)
                     {

com.unity.formats.fbx/Editor/ExportModelEditorWindow.cs

@@ -462,7 +462,7 @@ protected bool OverwriteExistingFile(string filePath){
             if (System.IO.File.Exists (filePath)) {
                 bool overwrite = UnityEditor.EditorUtility.DisplayDialog (
                     string.Format("{0} Warning", ModelExporter.PACKAGE_UI_NAME), 
-                    string.Format("File {0} already exists.\nOverwrite cannot be undone", filePath), 
+                    string.Format("File {0} already exists.\nOverwrite cannot be undone.", filePath), 
                     "Overwrite", "Cancel");
                 if (!overwrite) {
                     if (GUI.changed) {