GDSchool preprocessor: collect all errors and recap all at the end

[NathanLovato]

The latest version of gdschool_preprocess_mdx collects errors about missing media files and summarizes them at the end. It should ideally do so for all errors, including file reading errors. E.g. in the image size reading module, the errors "Cannot open the image file" are not surfaced at the end of a run.