Add special enum variant for errors from user code (#101)

As described in [JSON-RPC specs](https://www.jsonrpc.org/specification),
server errors must be used only for errors from server implementation.
They can take only values from -32099 to -32000, and library users must
not use them to describe business logic errors. I separated them from
user code errors, and now users must use MethodError to describe errors
from business logic