Custom type support feature

Hi,

I'm using the `rowserrcheck` plugin in a project and I'm looking for a way to get the same check for a custom iterator.

Example:
```
type CustomIter struct {}
func (*CustomIter) Err() error {...}
...
```

Would you be interested in extending this linter to accept a list of types to check in addition to the `Rows`? I can prepare a PR.