Try to have a useful message if provider failed

Author: bmalinowsky

src/tuwien/auto/calimero/serial/ConnectionFactory.java

@@ -1,6 +1,6 @@
 /*
     Calimero 2 - A library for KNX network access
-    Copyright (c) 2022, 2024 B. Malinowsky
+    Copyright (c) 2022, 2025 B. Malinowsky
 
     This program is free software; you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -79,7 +79,10 @@ public C open(final String name, final ThrowingFunction<P, C> openFunc) throws K
 				return Optional.of(conn);
 			}
 			catch (KNXException | IOException | RuntimeException | ExceptionInInitializerError t) {
-				logger.debug("{} unsuccessful: {}", provider, t.getMessage());
+				var msg = t.getMessage();
+				if (msg == null)
+					msg = t.getCause() != null ? t.getCause().getMessage() : t.getClass().getSimpleName();
+				logger.debug("{} unsuccessful: {}", provider, msg);
 				providerExceptions.add(t);
 				return Optional.<C>empty();
 			}