Remove restrictions on JSON-RPC error codes

The JSON-RPC spec (http://www.jsonrpc.org/specification) states:

1. The error code must be an integer
2. The error codes from and including -32768 to -32000 are reserved for
pre-defined errors.
3. The remainder of the space is available for application defined
errors.

The current behaviour only allows error codes in the reserved range
.i.e. the use of custom application defined errors is not allowed. This
is incorrect behaviour as according to the spec it is legitimate to
define and add error codes providing they are not in the reserved range.
This fix removes the restrictions on error codes.