Allow custom coercion failure messages

[zuk]

Currently if something goes wrong inside a custom param type's .parse method, the only feedback in the resulting validation failure response is a generic "____ is invalid" message. This pull request allows custom coercion failure messages by having the .parse method return an InvalidValue initialized with the failure message.

For example:

class Color
  attr_reader :value
  def initialize(color)
    @value = color
  end

  def self.parse(value)
    if  %w(blue red green).include?(value)
      new(value)
    else
      Grape::Validations::Types::InvalidValue.new "is not a valid color"
    end
  end
end

# ...

params do
  requires :color, type: Color, default: Color.new('blue')
end

get '/stuff' do
  # params[:color] is already a Color.
  params[:color].value
end

[dblock]

Great, can you squash these commits please?

[zuk]

Ok done. Not sure what's up with those failed Rubinius builds in Travis.

[dblock]

I am not in love with this. We're now explicitly checking what kind of exception we got. Any better suggestions?

[zuk]

I think there's a bigger problem here. Coercion and validation take separate paths but are really converging towards the same thing (i.e. we'll eventually end up needing full-fledged validation inside .parse). But this feels too monumental for a casual contributor to take on. Another way to do this would be to give custom types an optional .validate method that behaves like a regular custom validator, and when present, gets called prior to coercion. I considered doing it this way but thought the current proposal would be more likely to get pulled since it leverages the exiting BadValue type (which already seems a bit out of place... its acts like an exception, but isn't...)

[dblock]

I encountered something related in #1675. I think our validation + coercion needs some serious revisiting. Just thinking out loud.

[zuk]

I agree :) We've been using our own fork of Grape with the above change, periodically rebasing to origin. It works, but it's pretty hacky. There's probably a bigger architectural issue starting to surface here, but like I said before, it feels like too much for me to try to take on.