Improve error message for invalid classes

[tfausak]

This pull request is in response to #302. It changes the InvalidClassError message. If you define an object filter for a class that doesn't exist, the message can be confusing.

class Example < ActiveInteraction::Base
  object :cow
end
Example.run
# ActiveInteraction::InvalidClassError: "Cow"

The class is invalid to ActiveInteraction because it doesn't exist. The new error message is hopefully better.

Example.run
# ActiveInteraction::InvalidClassError: class "Cow" does not exist

[AaronLasseigne]

I like this.