Fix error codes to conform to the spec

[findleyr]

We are returning the "wrong" error code for "resource not found" because the spec incorrectly uses a code reserved for JSON-RPC implementions. (modelcontextprotocol/modelcontextprotocol#509). And our implementation actually does use that code. Since there has been no progress on that bug, we should probably change jsonrpc2_v2 and use the spec's code.