chainHead: Define error codes returned by the server

[lexnv]

This PR defined the error codes returned by the server:

• -32800 two or more subscriptions requested for chainHead_follow
• -32801 block hash not reported by chainHead_follow or block hash has been unpinned
• -32802 chainHead_follow started with withRuntime == false
• -32803 chainHead_follow did not generate an operationWaitingForContinue event

The following are errors defined in the JSON-RPC spec:

• -32602 The provided parameter isn't one of the expected values, has different format or is missing
• -32603 Internal server error:
  • The server was unable to generate a valid subscription ID (https://github.com/paritytech/polkadot-sdk/blob/21f1811c6600d8a7fe043592ff34dcb79284d583/substrate/client/rpc-spec-v2/src/chain_head/chain_head.rs#L198-L204)
  • The server cannot interpret the block: hardware error, ram/disk error, corrupt database, broken code logic (https://github.com/paritytech/polkadot-sdk/blob/21f1811c6600d8a7fe043592ff34dcb79284d583/substrate/client/rpc-spec-v2/src/chain_head/chain_head.rs#L306-L309)

This is done for consistency reasons, since at the moment the chainHead_follow is the only method that has an associated return code, although the other methods could still generate JSON-RPC errors.
This ensures interoperability as both substrate and light-client implementations will return similar codes.

cc @tomaka @jsdw @josepot @bkchr

[tomaka]

The server was unable to generate a valid subscription ID (https://github.com/paritytech/polkadot-sdk/blob/21f1811c6600d8a7fe043592ff34dcb79284d583/substrate/client/rpc-spec-v2/src/chain_head/chain_head.rs#L198-L204)

Note that this shouldn't be a legitimate error. Subscription IDs can be strings, so there is an infinite number of possible IDs. Even if it wasn't infinite, the spec mentions that there's a maximum number of simultaneous chainHead_follow subscriptions. I don't see in which world you can't generate a subscription ID.

[tadeohepperle]

👍

[lexnv]

Thanks for the reviews 🙏