Make errors accessable

This PR moves all calls to `errors.New()` into variables so a user of this library can more easily compare them, e.g. to avoid to call `Exists()` first.

```
item, err := cache.Value("somekey")
if err != nil {
  if err == cache2go.ErrKeyNotFound {
     // do something
  } 
  // Abort horrible
}
```