I/Q: Give names to error codes and align with semantics from JSON RPC spec

Remove hardcoded error codes such as -32603 from the I/Q implementation. Instead, introduce names for the various error code constants from the JSON RPC spec, and check that they are used as intended by the spec.

Start by locating the I/Q implementation sites that use hardcoded JSON-RPC error codes such as -32603, then consult the JSON-RPC specification for the named codes and their intended semantics. The work is done when the implementation uses named constants and the code paths match the specification's error meanings.

Written by the indexing model from the issue text.