exp/lighthorizon: consistent error formatting and semantics

[sreuland]

What problem does your feature solve?

• light horizon should re-use horizon problems package for overall consistent error message formatting on web service endpoints.
• error responses should have their http status code verified, correct 4XX or 5XX for the error context.
• For /accounts/{account_id}/... endpoints, if account_id doesn't exist in the underlying index, it returns empty response, is that acceptable or should it return 404, it's dependent on the exported index state, there is no current state of the network available that can provide the deterministic 'not found'

What would you like to see?

Good error descriptions and correct 4XX or 5XX http status codes on error responses from web endpoints.

What alternatives are there?

[Shaptic]

Closed by #4542.