Merge #18898: gui: Display warnings as rich text

a9d28afe23 qt: Display warnings as rich text (Hennadii Stepanov)

Pull request description:

  On master (6621be5351), warnings that contain `<hr />` HTML tag are not displayed correctly:

  ![Screenshot from 2020-05-06 11-30-10](https://user-images.githubusercontent.com/32963518/81177281-0e49fc80-8faf-11ea-8cac-8847aa517e86.png)

  Fixed:

  ![Screenshot from 2020-05-07 07-30-48](https://user-images.githubusercontent.com/32963518/81255618-ca9ad580-9036-11ea-90ad-7f4d89c1880d.png)

ACKs for top commit:
  jonasschnelli:
    utACK a9d28afe23
  promag:
    Code review ACK a9d28afe23.

Tree-SHA512: ba5b3837d5f6ea15c3255a3120c9753fc58ee67a370c388556214048ab993c45be720af7cb8d43bb0f12088956cb78abc77546ed1fc691082880438072fe774b

src/qt/bitcoin.cpp

@@ -399,7 +399,7 @@ void BitcoinApplication::shutdownResult()
 
 void BitcoinApplication::handleRunawayException(const QString &message)
 {
-    QMessageBox::critical(nullptr, "Runaway exception", BitcoinGUI::tr("A fatal error occurred. %1 can no longer continue safely and will quit.").arg(PACKAGE_NAME) + QString("\n\n") + message);
+    QMessageBox::critical(nullptr, "Runaway exception", BitcoinGUI::tr("A fatal error occurred. %1 can no longer continue safely and will quit.").arg(PACKAGE_NAME) + QString("<br><br>") + message);
     ::exit(EXIT_FAILURE);
 }