improve lists:nth/2 function, deal with exceed condition

lists:nth(N, L). when N>L.length, there will be an exception, not that friendly like:
```
lists:nth(1,[]).
** exception error: no function clause matching lists:nth(1,[]) (lists.erl, line 168)
```
I improve the nth/2 function to deal with exceed condition, which I think, can make it more friend to handle.
e.g.
```
mylist:nth(1, []).
error_exceed
```
thanks