show a warning if loading the font file failed

3rd_party/imgui/imgui_draw.cpp

@@ -1573,6 +1573,7 @@ ImFont* ImFontAtlas::AddFontFromFileTTF(const char* filename, float size_pixels,
     void* data = ImFileLoadToMemory(filename, "rb", &data_size, 0);
     if (!data)
     {
+        printf("failed to load font file: %s", filename);
         IM_ASSERT(0); // Could not load file.
         return NULL;
     }