Fix exception message

Author: vsilaev

src/main/java/net/tascalate/concurrent/AbstractCompletableTask.java

@@ -418,7 +418,7 @@ private static ExecutionException rewrapExecutionException(ExecutionException ex
         if (ex.getCause() instanceof CompletionException) {
             Throwable completionExceptionReason = ex.getCause().getCause();
             if (null != completionExceptionReason) {
-                return new ExecutionException(ex.getMessage(), completionExceptionReason);
+                return new ExecutionException(completionExceptionReason);
             }
         }
         return ex;