Show system error message on mmap() is failed

It is tested by changing grn_io_seg_map_() like:

     void
     grn_io_seg_map_(grn_ctx *ctx, grn_io *io, uint32_t segno, grn_io_mapinfo *info)
     {
    +   errno = ENOMEM;
    +   return;
        SEG_MAP(io, segno, info);
     }

NOTE: Windows version may not be worked. I didn't try it yet.