Add response code checking to the solution to exercise 20.1

Issue marijnh#185

Author: marijnh

code/solutions/20_1_content_negotiation_again.js

@@ -19,6 +19,10 @@ function readStreamAsString(stream, callback) {
     path: "/author",
     headers: {Accept: type}
   }, function(response) {
+    if (response.statusCode != 200) {
+      console.error("Request for " + type + " failed: " + response.statusMessage);
+      return;
+    }
     readStreamAsString(response, function(error, content) {
       if (error) throw error;
       console.log("Type " + type + ": " + content);