Faker: Improve err msg if XOpenDisplay !interposed

If a 3D application uses dlopen()/dlsym() to load X11 functions, then
libdlfaker.so must be preloaded.  Otherwise VirtualGL's interposed
version of XOpenDisplay() will not be called, the X11 display's
excluded status will not be added as a display extension, and
faker::isDisplayExcluded() and faker::getDisplayCS() will fail.  This
commit adds a more instructive error message if one of those methods
fails.

Refer to #214

Author: dcommander

server/faker.h

@@ -1,6 +1,6 @@
 // Copyright (C)2004 Landmark Graphics Corporation
 // Copyright (C)2005, 2006 Sun Microsystems, Inc.
-// Copyright (C)2009, 2011, 2013-2016, 2018-2022 D. R. Commander
+// Copyright (C)2009, 2011, 2013-2016, 2018-2022, 2025 D. R. Commander
 //
 // This library is free software and may be redistributed and/or modified under
 // the terms of the wxWindows Library License, Version 3.1 or (at your option)
@@ -95,8 +95,10 @@ namespace faker
 			XFindOnExtensionList(XEHeadOfExtensionList(obj), 0) ? 0 : 1;
 		extData = XFindOnExtensionList(XEHeadOfExtensionList(obj),
 			minExtensionNumber);
-		ERRIFNOT(extData);
-		ERRIFNOT(extData->private_data);
+		if(!extData || !extData->private_data)
+			THROW("VirtualGL extensions not found on X display.  If the 3D\n"
+				"[VGL]    application uses dlopen()/dlsym() to load X11 functions, then\n"
+				"[VGL]    VirtualGL's dlopen() interposer must be preloaded.");
 
 		return *(bool *)extData->private_data;
 	}
@@ -115,8 +117,10 @@ namespace faker
 			XFindOnExtensionList(XEHeadOfExtensionList(obj), 0) ? 0 : 1;
 		extData = XFindOnExtensionList(XEHeadOfExtensionList(obj),
 			minExtensionNumber + 1);
-		ERRIFNOT(extData);
-		ERRIFNOT(extData->private_data);
+		if(!extData || !extData->private_data)
+			THROW("VirtualGL extensions not found on X display.  If the 3D\n"
+				"[VGL]    application uses dlopen()/dlsym() to load X11 functions, then\n"
+				"[VGL]    VirtualGL's dlopen() interposer must be preloaded.");
 
 		return *(util::CriticalSection *)extData->private_data;
 	}