RPC input validation

RPC endpoints where the input data fails validation should return error 422 Unprocessable Entity, rather than:

• returning null, e.g. chains/main/blocks/head/context/contracts/tz1id9tkesR9k4qW8WwH9cPXykLRmrCS5Vit/manager_key
• 404 (Not_found), e.g. chains/main/blocks/head/context/contracts/tz1id9tkesR9k4qW8WwH9cPXykLRmrCS5Vit/delegate and chains/main/blocks/head/context/contracts/tz1id9tkesR9k4qW8WwH9cPXykLRmrCS5Vit/delegate_opt
• 500 internal server error, e.g. chains/main/blocks/head/context/delegates/tz1id9tkesR9k4qW8WwH9cPXykLRmrCS5Vit/balance

where tz1id9tkesR9k4qW8WwH9cPXykLRmrCS5Vit is not allocated (has no balance on chain)