Error handling

[NikkiBytes]

Upgraded with comprehensive error catching in the validation endpoint and schema metadata endpoint.
Added a a test file that tests scenarios in the validation endpoint.

[NikkiBytes]

@newgene just pushed a fourmat check update, this has been reviewed and is ready to merge.

[newgene]

@NikkiBytes just to make sure we (you, @marcodarko and I) are on the same page about the error-handling behavior, can you please document all handled cases in this PR. A table looks like this might be an option:

is schema.org class	class exists	class contains validation	...	/schema/<id> status code	/schema/<id>/validation status code
✅	✅			200	404

[NikkiBytes]

#	Is schema.org class/property	Is registered schema class/property	Class contains validation	/schema/<id> status code	/schema/<id>/validation status code
1	❌	❌	❌	404 (Unknown/invalid schema id)	404 (Unknown/invalid schema id)
2	❌	✅	❌	200	404(validation field not found)
3	❌	✅	✅	200	200
4	✅	❌	❌	404 (Metadata from schema.org cannot be retrieved this way)	404