Fixed issues with blend shape animations (#3080)

Duplicate blend shape animations are now handled correctly.
Invalid animation targets are now an error instead of a crash in the builder.

Signed-off-by: stankowi <4838196+AMZN-stankowi@users.noreply.github.com>

Code/Tools/SceneAPI/SDKWrapper/AssImpSceneWrapper.cpp

@@ -70,6 +70,9 @@ namespace AZ
             // This results in the loss of the offset matrix data for nodes without a mesh which is required for the Transform Importer.
             m_importer.SetPropertyBool(AI_CONFIG_IMPORT_FBX_PRESERVE_PIVOTS, false);
             m_importer.SetPropertyBool(AI_CONFIG_IMPORT_FBX_OPTIMIZE_EMPTY_ANIMATION_CURVES, false);
+            // The remove empty bones flag is on by default, but doesn't do anything internal to AssImp right now.
+            // This is here as a bread crumb to save others times investigating issues with empty bones.
+            // m_importer.SetPropertyBool(AI_CONFIG_IMPORT_REMOVE_EMPTY_BONES, false);
             m_sceneFileName = fileName;
             m_assImpScene = m_importer.ReadFile(fileName,
                 aiProcess_Triangulate //Triangulates all faces of all meshes