gui: Show messages as text not html

Currently, error messages (such as InitError) are displayed as-is, which
means Qt does auto detection on the format.

This means that it's possible to inject HTML from the command line
though e.g. specifying a wallet name with HTML in it. This isn't
a direct security risk because fetching content from internet is
disabled (and as far as I know we never report strings received
from the network this way). However, it can be confusing.

So explicitly force the format as text.

src/qt/bitcoingui.cpp

@@ -923,6 +923,7 @@ void BitcoinGUI::message(const QString &title, const QString &message, unsigned
 
         showNormalIfMinimized();
         QMessageBox mBox(static_cast<QMessageBox::Icon>(nMBoxIcon), strTitle, message, buttons, this);
+        mBox.setTextFormat(Qt::PlainText);
         int r = mBox.exec();
         if (ret != nullptr)
             *ret = r == QMessageBox::Ok;