Raise RPC_WALLET_NOT_SPECIFIED instead of RPC_METHOD_NOT_FOUND when a required
wallet filename was not specified in an RPC call.
Also raise more specific RPC_WALLET_NOT_FOUND error instead of
RPC_INVALID_PARAMETER in case an invalid wallet was specified, for consistency.
RPC_INVALID_REQUEST and RPC_METHOD_NOT_FOUND are mapped internally to
HTTP error codes and should not be used for application-layer errors.
This commit adds commenting around those definitions to warn not to use
them for application errors.