clarify missing field error message when using an Array for the with_same option

This PR clarifies the error message you receive when supplying an Array of fields to the `with_same`: 
```ruby
ranks :row_order, with_same: [:name, :non_existant_field]
```
which currently returns this message:
```
No field called "[:name, :non_existant_field]" found in model
```
But now you receive a precise error detailing which field is problematic:
```
No field called "non_existant_field" found in model
```