Make error a predefined variable for later comparision

https://github.com/go-chi/render/blob/3215478343fbc559bd3fc08f7031bb134d6bdad5/decoder.go#L30

If the returned error are defined var, it would be possible to compare and know what exactly happened. The error message from render package cannot be directly to client in most cases, but application developer would like to return some custom error.

```golang
var (
ErrContentTypeNotSet = errors.New("render: missing content-type, unable to automatically decode the request content")
)
```
would be helpful.