Fix MyGUI log

Also actually print an error to the regular log when the MyGUI log can't
be opened so we notice if we kill it again in under five years.

apps/openmw/mwgui/windowmanagerimp.cpp

@@ -189,7 +189,7 @@ namespace MWGui
     {
         float uiScale = Settings::Manager::getFloat("scaling factor", "GUI");
         mGuiPlatform = new osgMyGUI::Platform(viewer, guiRoot, resourceSystem->getImageManager(), uiScale);
-        mGuiPlatform->initialise(resourcePath, logpath);
+        mGuiPlatform->initialise(resourcePath, (boost::filesystem::path(logpath) / "MyGUI.log").generic_string());
 
         mGui = new MyGUI::Gui;
         mGui->initialise("");

components/myguiplatform/myguiloglistener.cpp

@@ -2,11 +2,15 @@
 
 #include <iomanip>
 
+#include <components/debug/debuglog.hpp>
+
 namespace osgMyGUI
 {
     void CustomLogListener::open()
     {
         mStream.open(boost::filesystem::path(mFileName), std::ios_base::out);
+        if (!mStream.is_open())
+            Log(Debug::Error) << "Unable to create MyGUI log with path " << mFileName;
     }
 
     void CustomLogListener::close()