Fixed duplicate case compile error

I tried to compile Cockatrice and found the following error:

Cockatrice/cockatrice/src/window_main.cpp: In member function 'void MainWindow::loginError(Response::ResponseCode, QString, quint32)':
Cockatrice/cockatrice/src/window_main.cpp:261:3: error: duplicate case value
Cockatrice/cockatrice/src/window_main.cpp:249:3: error: previously used here

I looked at the source and I deduced from the "Invalid username" codeline and the file common/pb/response.proto that the second "case Response::RespUserIsBanned" should actually be a "case Response::RespUsernameInvalid" case.
If this is correct, please apply my fix.