Fixed panic message in IgnoreUnexported

[ko30005]

Fixed panic message more specifically.
With this fix, you can see that IgnoreUnexported only supports a struct without the user having to look at the code.

[dsnet]

Thanks for the PR.

For consistency, we probably want to adjust this panic message too:

go-cmp/cmp/cmpopts/struct_filter.go

Lines 43 to 46 in d669b04

	t := reflect.TypeOf(typ)
	if t == nil || t.Kind() != reflect.Struct {
	panic(fmt.Sprintf("%T must be a struct", typ))
	}

[ko30005]

Thanks!
I also fixed this panic message.

[dsnet]

Thanks!