Check for unrecognised custom part types when uploading questions

I uploaded a question I'd downloaded from the mathcentre editor to my local instance. It used a custom part type that I haven't got on my instance, so failed to compile.

The upload view should check the types of all the parts in uploaded questions, and throw an error if there are any unrecognised types.

Start at the upload view and trace how uploaded questions and their parts are checked before compilation. Done means an uploaded question containing an unrecognised custom part type produces a clear error instead of failing during compilation.

Written by the indexing model from the issue text.