Separate message and exception by newline (#1055)

This was a small discrepancy to the output of vanilla Maven and made the output
hard to read, since the failure class name started right after the message without
any whitespace in between.

logging/src/main/java/org/slf4j/impl/MvndSimpleLogger.java

@@ -168,6 +168,7 @@ protected void writeThrowable(Throwable t, StringBuilder sb) {
             return;
         }
         MessageBuilder builder = MessageUtils.buffer(sb);
+        builder.newline();
         builder.failure(t.getClass().getName());
         if (t.getMessage() != null) {
             builder.a(": ");