Add well-formed timetable validation check

Right now, NUSMods just checks on load for whether a timetable object exists. This could potentially cause issues since we never explicitly check that data is of type PageData. Notice here that data is implicitly converted from any as returned in JSON.parse to PageData, received as a parameter to validatePageData.

nusmods/export/src/data.ts

Lines 49 to 50 in 47238fa

    
           const data = JSON.parse(ctx.query.data); 
        
           validateExportData(data);

nusmods/export/src/data.ts

Lines 66 to 71 in 089993f

    
           // TODO: Improve these validation 
        
           if (!_.isObject(data.timetable)) { 
        
             throw new Error('Invalid timetable'); 
        
           }

We could implement a basic check that the data it contains is well-formed according to PageData's type definition.

	const data = JSON.parse(ctx.query.data);
	validateExportData(data);


	// TODO: Improve these validation
	if (!_.isObject(data.timetable)) {
	throw new Error('Invalid timetable');
	}