Better log message / error format

[yookoala]

Errors return by CaseHandler should give a better idea of what the error is about.

An error should include:

1. The endpoint URL description (what).
2. The method that applied to the endpoint (action).
3. The payload, if any, sent to the endpoint (action details).
4. The expectation (what is the expected result).
5. The result (what is the actual result).
6. An error involves a JSON should also describe clearly the path of the expected value / the result.