return failure if rewriter cannot rewrite properly

[yonghong-song]

Fix issue #537.

The bcc rewriter does not have enough information to do
proper rewriting from:

  #define PKT_LEN_ADD 1
  ip->tlen += PKT_LEN_ADD;

to

  bpf_dins_pkt(skb, (u64)ip+2, 0, 16, PKT_LEN_ADD);

So instead of generate incorrect code
which caused compilation error. Let return an error
earlier with helper comments so users know what to do.
With this patch, we will have

 /virtual/main.c:20:17: error: cannot have macro at the end of expresssion,
 workaround: put perentheses around macro "(MARCO)"
    ip->tlen += PKT_LEN_ADD;
                ^