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

[marcodarko]

@NikkiBytes can you review this PR? is this still relevant or has this been addressed?

[NikkiBytes]

@marco are you able to see where the codacy failed?

[NikkiBytes]

for class/property, e.g. schema:Dataset, n3c:Dataset

schema.org	class/property exists	has_validation	/api/schema/<curie> status	/api/schema/validation status
No	no	no	404	404
No	yes	no	200	404
No	yes	yes	200	200
Yes	yes	no	200	404

for ns:

schema.org	namespace exists	/api/schema/<ns> status
no	no	404
no	yes	200
yes	yes	200 [with special returned JSON]

400 for any other input value issue

• a:b:c invalid curie
• a,b:dataset invalid.