qt: Prevent non-functional GUI from popping up during Init

When a InitError or InitWarning happens, the
GUI pops up but is unusable (until Init finishes).

This is caused by showNormalIfMinimized. Add a message
flag to skip this call for Init errors or warnings.

src/init.cpp

@@ -164,13 +164,13 @@ void HandleSIGHUP(int)
 
 bool static InitError(const std::string &str)
 {
-    uiInterface.ThreadSafeMessageBox(str, "", CClientUIInterface::MSG_ERROR);
+    uiInterface.ThreadSafeMessageBox(str, "", CClientUIInterface::MSG_ERROR | CClientUIInterface::NOSHOWGUI);
     return false;
 }
 
 bool static InitWarning(const std::string &str)
 {
-    uiInterface.ThreadSafeMessageBox(str, "", CClientUIInterface::MSG_WARNING);
+    uiInterface.ThreadSafeMessageBox(str, "", CClientUIInterface::MSG_WARNING | CClientUIInterface::NOSHOWGUI);
     return true;
 }
 

src/qt/bitcoingui.cpp

@@ -681,7 +681,10 @@ void BitcoinGUI::message(const QString &title, const QString &message, unsigned
         if (!(buttons = (QMessageBox::StandardButton)(style & CClientUIInterface::BTN_MASK)))
             buttons = QMessageBox::Ok;
 
-        // Ensure we get users attention
+        // Ensure we get users attention, but only if main window is visible
+        // as we don't want to pop up the main window for messages that happen before
+        // initialization is finished.
+        if(!(style & CClientUIInterface::NOSHOWGUI))
             showNormalIfMinimized();
         QMessageBox mBox((QMessageBox::Icon)nMBoxIcon, strTitle, message, buttons, this);
         int r = mBox.exec();

src/ui_interface.h

@@ -62,6 +62,8 @@ public:
 
         /** Force blocking, modal message box dialog (not just OS notification) */
         MODAL               = 0x10000000U,
+        /** Don't bring GUI to foreground. Use for messages during initialization */
+        NOSHOWGUI           = 0x20000000U,
 
         /** Predefined combinations for certain default usage cases */
         MSG_INFORMATION = ICON_INFORMATION,