parse RPC errors into exception object and provide better output handling

[nikcub]

parse JSON-RPC error object into code and message and provide string and object casting.

example:

try:
    block_hashes = rpc_connection.batch_(commands)
    blocks = rpc_connection.batch_([ [ "getblocks", h ] for h in block_hashes ])
    block_times = [ block["time"] for block in blocks ]
    print(block_times)
except JSONRPCException, e:
    print "Error: %s" % e
    print e.code
    print e.message

backwards compatible.