Add more context to the error messages

Errors like:

fio: pid=0, err=22/file:filesetup.c:380, error=Invalid argument

do not give a lot of clue as to what is wrong, unless you
have a matching source. So add a context relevant info
message as well.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index c938960..8141991 100644
--- a/io_u.c
+++ b/io_u.c
@@ -548,7 +548,7 @@
 	if (min_events > 0) {
 		ret = td_io_commit(td);
 		if (ret < 0) {
-			td_verror(td, -ret);
+			td_verror(td, -ret, "td_io_commit");
 			return ret;
 		}
 	} else {
@@ -559,7 +559,7 @@
 
 	ret = td_io_getevents(td, min_events, td->cur_depth, tvp);
 	if (ret < 0) {
-		td_verror(td, -ret);
+		td_verror(td, -ret, "td_io_getevents");
 		return ret;
 	} else if (!ret)
 		return ret;