qt: Display warnings as rich text
qt: Display warnings as rich text

Summary: This is a backport of [[https://github.com/bitcoin/bitcoin/pull/18898 | core#18898]]

Test Plan:
```
diff --git a/src/qt/bitcoin.cpp b/src/qt/bitcoin.cpp
index dd46393259..62fef6c7bd 100644
--- a/src/qt/bitcoin.cpp
+++ b/src/qt/bitcoin.cpp
@@ -789,6 +789,9 @@ int GuiMain(int argc, char *argv[]) {
             QObject::tr("%1 didn't yet exit safely...").arg(PACKAGE_NAME),
             (HWND)app.getMainWinId());
 #endif
+        app.handleRunawayException(
+            QString::fromStdString("<hr />Fake warning"));
         app.exec();
         app.requestShutdown(config);
         app.exec();
```

Compile and run `bitcoin-qt`

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9932

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