rpc: support wrapped errors and code-based Is for jsonError

[pmikolajczyk41]

1. errorMessage previously cast err directly to rpc.Error/DataError, which missed errors wrapped with fmt.Errorf("%w", ...). Switch to errors.As so the code and data are extracted correctly from wrapped errors.
2. Add (*jsonError).Is so that errors.Is(receivedErr, sentinel) works on the client side when receivedErr is a jsonError reconstructed from the wire. Matching is by error code: if the sentinel implements ErrorCode() int, the codes are compared. This avoids the need for string-based error reconstruction in callers.

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pulled in by OffchainLabs/nitro#4629