fixed log message output for WoT validation errors

Author: thjaeckle

wot/api/src/main/java/org/eclipse/ditto/wot/api/validator/DefaultWotThingModelValidator.java

@@ -612,7 +612,7 @@ private static void logValidationWarning(final boolean logAsWarning,
         final LoggingEventBuilder logBuilder = logAsWarning ? log.atWarn() : log.atInfo();
         logBuilder.log("WoT based validation of Thing <{}> in <{}()> failed for <TD {}>/<FD {}> due to: <{}>",
                 context.thingId(), loggingHintSource, context.thingDefinition(), context.featureDefinition(),
-                throwable.getMessage()
+                throwable.toString()
         );
         dittoHeaders.getCorrelationId().ifPresent(cId -> MDC.remove("correlation-id"));
         dittoHeaders.getTraceParent().ifPresent(traceParent -> MDC.remove("traceparent-trace-id"));