Don't overwrite error

The first error is usually the most interesting, so don't overwrite
in __td_verror().

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 304f658..aeb45bf 100644
--- a/fio.h
+++ b/fio.h
@@ -425,6 +425,8 @@
 
 #define __td_verror(td, err, msg)					\
 	do {								\
+		if ((td)->error)					\
+			break;						\
 		int e = (err);						\
 		(td)->error = e;					\
 		snprintf(td->verror, sizeof(td->verror) - 1, "file:%s:%d, error=%s", __FILE__, __LINE__, (msg));	\