Respect superclass's initializer arity in Error

This fixes rspec-mocks and probably other mocking libs that create instances of
error classes without arguments.

E.g. in rspec-mocks `api.stub(:foobar).and_raise(Octokit::NotFound)` would cause
an error because of the missing response argument in Octokit::Error#initializer.
Merge pull request #213 from fphilipe/fix-error-class

Respect superclass's initializer arity in Error