Do not repeat the same error messages

[kiela]

If you want to validate param if:

• is an integer
• is greater than 1

and you have in your code something similar to:

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:

years is invalid, years is invalid

This PR fixes this this behavior and makes grape to return the same error message only once.

[dblock]

Thanks. I know it's trivial, but this needs a test and an update to CHANGELOG, please.

[dblock]

Thanks, merging.