net: fix inconsistent error values on Read for solaris

Updates #4856.

Change-Id: Ia04e24fb1fe57e244d7b1cd417f7f419ad610acd
Reviewed-on: https://go-review.googlesource.com/9776
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>

src/net/sendfile_solaris.go

@@ -103,5 +103,8 @@ func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
 	if lr != nil {
 		lr.N = remain
 	}
+	if err != nil {
+		err = os.NewSyscallError("sendfile", err)
+	}
 	return written, err, written > 0
 }