[FIX] fix connection error handling

if connection to server can't be established, reject Promise of `connect()` method caller,
instead of throwing error event on base object.

test/connection-test.js

@@ -106,6 +106,26 @@ describe('Connection', function() {
     });
   });
 
+  it('should throw NotConnectedError if server not responding ', function(
+    done
+  ) {
+    if (process.browser) {
+      const phantomTest = /PhantomJS/;
+      if (phantomTest.test(navigator.userAgent)) {
+        // inside PhantomJS this one just hangs, so skip as not very relevant
+        done();
+        return;
+      }
+    }
+
+    const connection = new utils.common.Connection('ws://127.0.0.1:321');
+    connection.on('error', done);
+    connection.connect().catch(error => {
+      assert(error instanceof this.api.errors.NotConnectedError);
+      done();
+    });
+  });
+
   it('DisconnectedError', function() {
     this.api.connection._send(JSON.stringify({
       command: 'config',