- Added `rpcPluginError` to encapsulate plugin errors with HTTP status codes.
- Enhanced `decodeEnvelopeResult` to preserve and return detailed plugin errors with status codes.
- Introduced `isPluginErrorEnvelope` to identify plugin error envelopes.
- Updated plugin call logic in Unix and Windows loaders to differentiate plugin errors from system errors.
- Added unit tests to verify error handling and status code preservation.