Updated invalid username dlg

Relates to #964. We want to give a the correct reason for failed logins
and display the reason why.

cockatrice/src/window_main.cpp

@@ -78,7 +78,7 @@ void MainWindow::processConnectionClosedEvent(const Event_ConnectionClosed &even
             break;
         }
         case Event_ConnectionClosed::SERVER_SHUTDOWN: reasonStr = tr("Scheduled server shutdown."); break;
-        case Event_ConnectionClosed::USERNAMEINVALID: reasonStr = tr("Invalid username."); break;
+        case Event_ConnectionClosed::USERNAMEINVALID: reasonStr = tr("Invalid username.\nAvailable chars: [a-z][A-Z][0-9][-_]"); break;
         default: reasonStr = QString::fromStdString(event.reason_str());
     }
     QMessageBox::critical(this, tr("Connection closed"), tr("The server has terminated your connection.\nReason: %1").arg(reasonStr));
@@ -261,7 +261,7 @@ void MainWindow::loginError(Response::ResponseCode r, QString reasonStr, quint32
             break;
         }
         case Response::RespUsernameInvalid:
-            QMessageBox::critical(this, tr("Error"), tr("Invalid username."));
+            QMessageBox::critical(this, tr("Error"), tr("Invalid username.\nAvailable chars: [a-z][A-Z][0-9][-_]"));
             break;
         case Response::RespRegistrationRequired:
             QMessageBox::critical(this, tr("Error"), tr("This server requires user registration."));