Implement custom JSON-RPC Errors.

Currently when a Modbus server returns an Modbus error code, Tolk returns a JSON-RPC error with code -32603: Internal JSON-RPC error containing the stacktrace. Response below fails with Modbus exception code 2: Illegal Data Address.

{
    "error": {
        "code": -32603,
        "data": "Traceback (most recent call last):\n  File \"/home/auke/projects/tolk/.env/local/lib/python2.7/site-packages/pyjsonrpc/rpclib.py\", line 115, in call\n    result = rpc_function(*positional_params, **named_params)\n  File \"/home/auke/projects/tolk/scripts/../tolk/json_rpc.py\", line 25, in read_coils\n    int(quantity))\n  File \"/home/auke/projects/tolk/.env/local/lib/python2.7/site-packages/modbus_tk/utils.py\", line 26, in new\n    raise excpt\nModbusError: Modbus Error: Exception code = 2\n",
        "message": "Internal JSON-RPC error."
    },
    "id": 246344073102444981514353044232564665595,
    "jsonrpc": "2.0"
}

The JSON-RPC specification leaves room for custom errors. Below the surface Tolk uses Modbus-TK which implements 7 Modbus error codes. Tolk should return custom JSON-RPC errors reflecting the Modbus error code that was returned. Tolk should implement the following error codes:

[ ] 1: Illegal Function as JSON-RPC error -32001: Illegal Function
[ ] 2: Illegal Data Address as JSON-RPC error -32002: Illegal Data Address
[ ] 3: Illegal Data Value as JSON-RPC error -32003: Illegal Data Value
[ ] 4: Slave Device Failure as JSON-RPC error -32004: Slave Device Failure
[ ] 5: Command Acknowledge as JSON-RPC error -32005: Command Acknowledge
[ ] 6: Slave Device Busy as JSON-RPC error -32006: Slave Device Busy
[ ] 8: Memory Parity Error as as JSON-RPC error -32008: Memory Parity Error

An error response now could look like this:

{
    "error": {
        "code": -32001,
        "data": "Function code 27 is not  valid."
    },
    "id": 246344073102444981514353044232564665595,
    "jsonrpc": "2.0"
}

AdvancedClimateSystems / Tolk

Implement custom JSON-RPC Errors. #2