feat: propagate `nim-json-rpc` exceptions as `EthersNetworkError`

[emizzle]

Currently, nim-ethers converts all nim-json-rpc exceptions into JsonRpcProviderError. This should be modified to convert them into an ethers-specific exception type, eg EthersNetworkError. The other option is to bubble up the errors as is but this will create a double-handling situation, where those specific errors need to be treated differently at both the ethers level and the codex level.

Network-related errors to handle (with their corresponding nim-json-rpc error types):

• connection timed out (RpcPostError)
• connection closed during response (FailedHttpResponse)
• HTTP 429 "too many requests" (ErrorResponse(status=429))

See logos-storage/logos-storage-nim#1230 for more info.