fix(browser): nicer "disconnect" - no more "Disconnectedundefined"

The method "disconnect" sometimes is called without arguments. The emited message should not look broken then.

lib/browser.js

@@ -37,7 +37,7 @@ var Browser = function (id, fullName, /* capturedBrowsers */ collection, emitter
     self.state = DISCONNECTED
     self.disconnectsCount++
     log.warn('Disconnected (%d times)' + (reason || ''), self.disconnectsCount)
-    emitter.emit('browser_error', self, 'Disconnected' + reason)
+    emitter.emit('browser_error', self, 'Disconnected' + (reason || ''))
     collection.remove(self)
   }