Enrolling to a CA that returns an empty response crashes the node process

If a CA server responds with an empty response during enrollment, an exception is thrown in the event emitter on `end`. This is an uncaughtException and causes the containing node process to exit. # To replicate: With the attached files, run: ``` npm install node badCa.js & node index.js ``` This starts a simple malicious CA server that provides a response that triggers the exception. The `index.js` script tries 2 different scenarios: - 1. With a known bad URL, which throws an exception which can be caught by the client application - 2. With a malicious CA, which throws an exception which cannot be caught by the client application because of the EventEmitted behaviour in node. > From https://nodejs.org/api/events.html#events_error_events > If an EventEmitter does not have at least one listener registered for the 'error' event, and an 'error' event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits. # Fix Replace https://github.com/hyperledger/fabric-sdk-node/blob/c10865cfb20d063fdef4c7d96c25c1581f309e84/fabric-ca-client/lib/FabricCAClient.js#L457 `util.format('Enrollment failed with HTTP status code', JSON.parse(data).statusCode)));` with `util.format('Enrollment failed with HTTP status code', response.statusCode)));` The same issue is present at https://github.com/hyperledger/fabric-sdk-node/blob/0d24cec10f7a3e0153fcf3f0158a89c5eaa0cfab/fabric-ca-client/lib/FabricCAClient.js#L318 ## Impact This attack allows an attacker to cause any node client application using the fabric-ca-node SDK to exit. In scenarios where the SDK is used in a server-side application, this will have the effect of denying the use of that server to other users.