fix a bug in com.mogujie.tt.ui.activity.LoginActivity.java

In previous version, if we just input username, then click "login" button with password empty, there will come out a toast showing "please input username". it is obviously not suitable.

I just add a string named "error_password_required" to show "please input password" in the above case.

In my version, to be frankly, if we leave both "username" and "password" empty, then click "login" button, there will come out "please input password". it is not that good, and we should change the logic here. it may need adjust the logic and i am not sure what you want, so leave it to you.