Fix: mutually_exclusive and exactly_one_of validation error messages now label parameters as strings, consistently with requires and optional.

I was getting

```
"[:user_id, :profile_id]" => ["- exactly one parameter must be provided"]
```

but at the same time

```
"user_id" => ["is missing"]
```

This also enables returning JSON, ie. not translating an array of parameters into a string.