Add per-method errors to openrpc.json spec

[r-near]

Context

The errors module (src/errors.rs) now contains hand-written per-method RPC error enums (RpcQueryError, RpcBlockError, RpcTransactionError, RpcValidatorError) that match nearcore's chain/jsonrpc-primitives types. These were written by hand because the OpenRPC spec (openrpc.json) currently has no method-level error definitions — every method shows "no errors defined".

Goal

Add errors fields to each method definition in openrpc.json so these types can be auto-generated by the build script instead of maintained manually. The error shapes follow nearcore's #[serde(tag = "name", content = "info", rename_all = "SCREAMING_SNAKE_CASE")] convention.

Methods that need errors defined

• query methods → RpcQueryError (13 variants)
• block / chunk → RpcBlockError (3 variants)
• send_tx / broadcast_tx_commit / broadcast_tx_async / tx → RpcTransactionError (6 variants)
• validators → RpcValidatorError (3 variants)

Reference

• Hand-written enums: src/errors.rs in this repo
• nearcore source of truth: chain/jsonrpc-primitives/src/types/ in nearcore