Add on('error') to http request example

for indexzero

doc/api/http.markdown

@@ -396,6 +396,10 @@ Example:
       });
     });
 
+    req.on('error', function(e) {
+      console.log('problem with request: ' + e.message);
+    });
+
     // write data to request body
     req.write('data\n');
     req.write('data\n');