Do not repeat the same error messages

If you want to validate param if:
 - is an integer
 - is greater than 1

and you have in your code something similar to:
```ruby
params do
  requires :years, type: Integer, desc: "number of years", regexp: /^([1-9]\d+)/
end
```

and request contains `years` set to `1` or `-1` or `foo`, `grape` produces output with the same message twice:
```ruby
years is invalid, years is invalid
```

This PR fixes this this behavior and makes `grape` to return the same error message only once.