Fixed crash when loading invalid 3MFs (SPE-2135)

The crash was introduced in 040a846, which completely threw away handling of the return value
of the importer.load_model_from_file function and replaced it with weaker condition. The purpose
of that change was to solve #8401 (missing error message when loading different invalid 3MF).

This commit reverts that change (and reintroduces #8401). Handling of 3MF loading errors
should be inside the importer.load_model_from_file function, where the check should be
added later.

src/libslic3r/Format/3mf.cpp

@@ -3574,11 +3574,11 @@ bool load_3mf(const char* path, DynamicPrintConfig& config, ConfigSubstitutionCo
     // All import should use "C" locales for number formatting.
     CNumericLocalesSetter locales_setter;
     _3MF_Importer         importer;
-    importer.load_model_from_file(path, *model, config, config_substitutions, check_version);
+    bool res = importer.load_model_from_file(path, *model, config, config_substitutions, check_version);
     importer.log_errors();
     handle_legacy_project_loaded(importer.version(), config, importer.prusaslicer_generator_version());
 
-    return !model->objects.empty() || !config.empty();
+    return res;
 }
 
 bool store_3mf(const char* path, Model* model, const DynamicPrintConfig* config, bool fullpath_sources, const ThumbnailData* thumbnail_data, bool zip64)