API error response should be clearly distinguishable from the successful one

[azasypkin]

Based on #520 (comment)

it's a bit hard to distinguish successful response with value and failed one (especially for batch requests where response code should be 200 in any case):

• {"OpenZWave-d32394d4-01000000024c0000":"Closed"};
• {"OpenZWave-d32394d4-01000000024c0000":{"Error":{"OperationNotSupported": ...}}

Currently we rely on Error property of returned object, but that's dumb :) Theoretically valid value can be object with Error property. Something like this {"OpenZWave-d32394d4-01000000024c0000":{ value: "Closed", error: {"OperationNotSupported": ...}} would probably be less confusing.