asset load flow work

add reloading state to asset tracking,
convert most asset.isnull() checks on over to skipping out unless the errcode is ::Ok, or ::UsingFallback
add more errstriongcodes to hook up
skip load<type> execution if the asset loaded state is ::Ok

Engine/source/assets/assetBase.cpp

@@ -58,6 +58,7 @@ const String AssetBase::mErrCodeStrings[] =
    "Failed",
    "Ok",
    "NotLoaded",
+   "Reloading",
    "BadFileReference",
    "InvalidFormat",
    "DependencyNotFound",
@@ -293,6 +294,8 @@ void AssetBase::refreshAsset(void)
    if (mpOwningAssetManager == NULL || !mAssetInitialized)
       return;
 
+   mLoadedState = Reloading;
+
    // Yes, so refresh the asset via the asset manager.
    mpOwningAssetManager->refreshAsset(getAssetId());
 }