WIP Refactoring of exceptions:

1) All slicer's exceptions are now derived from Slic3r::Exception.
2) New exceptions are defined for slicing errors.
3) Exceptions are propagated to the Plater to show.
It remains to modify the slicing back-end to throw the new SlicingError
exceptions instead of std::runtime_error and to show the other exceptions
by a message dialog instead of a notification.

src/slic3r/GUI/GUI_App.cpp

@@ -571,7 +571,7 @@ void GUI_App::init_app_config()
         std::string error = app_config->load();
         if (!error.empty())
             // Error while parsing config file. We'll customize the error message and rethrow to be displayed.
-            throw std::runtime_error(
+            throw Slic3r::RuntimeError(
                 _u8L("Error parsing PrusaSlicer config file, it is probably corrupted. "
                     "Try to manually delete the file to recover from the error. Your user profiles will not be affected.") +
                 "\n\n" + AppConfig::config_path() + "\n\n" + error);