accept -3 as error return from the callback, too

io/io_mmapwritefile.c

@@ -55,7 +55,7 @@ int64 io_mmapwritefile(int64 out,int64 in,uint64 off,uint64 bytes,io_write_callb
       if (left>bytes) left=bytes;
       while (left>0) {
 	m=writecb(out,c,left);
-	if (m==-1) {
+	if (m<0) {
 	  e->canwrite=0;
 	  e->next_write=-1;
 	  if (errno!=EAGAIN) {