Mono: Show custom error explaining that UWP is not supported

Fixes #64490.

(cherry picked from commit ceef056146b62d9580ab600fbf453e386023bf1d)

platform/uwp/export/export.cpp

@@ -29,6 +29,7 @@
 /*************************************************************************/
 
 #include "export.h"
+
 #include "core/bind/core_bind.h"
 #include "core/crypto/crypto_core.h"
 #include "core/io/marshalls.h"
@@ -42,6 +43,12 @@
 #include "editor/editor_node.h"
 #include "platform/uwp/logo.gen.h"
 
+// Mono build doesn't support UWP, so we show a specific error.
+// We don't bypass the whole logic so that it doesn't lose potential UWP presets
+// added to export_presets.cfg from a non-Mono build (and in case third-parties
+// actually have Mono-enabled UWP templates they can use).
+#include "modules/modules_enabled.gen.h" // For mono.
+
 #include "thirdparty/minizip/unzip.h"
 #include "thirdparty/minizip/zip.h"
 
@@ -1110,6 +1117,16 @@ public:
 		}
 
 		valid = dvalid || rvalid;
+
+#ifdef MODULE_MONO_ENABLED
+		// If this is a Mono build, provide a custom error so that users are not confused.
+		// We don't bypass the whole logic to check templates because third-parties might have
+		// Mono-enabled UWP builds using this path.
+		r_missing_templates = false; // Don't warn about those.
+		r_error = TTR("Godot's Mono version does not support the UWP platform. Use the standard build (no C# support) if you wish to target UWP.");
+		return false;
+#endif // MODULE_MONO_ENABLED
+
 		r_missing_templates = !valid;
 
 		// Validate the rest of the configuration.