Collectors must Respond with JSON

[VincenzoFerme]

Apply the same improvement applied for the monitors so that all the responses to client are in structured JSON objects, where the structure should be placed in the commons package to the collectors. The structure can simply be:

{
   status: "SUCCESS" or "FAILED"
   message: "..."
}

Of course the same information should be printed on the standard output of the collectors. This applies to all the collectors, because everything must be logged and the client should be aware about what happens in the collectors.

Start from the changes made in #89 and improve the error handling, the structure of the returned message, the use of http errors in case of internal error and so on.