Support permitted error types

[searls]

Some code raises error intentionally for certain inputs and is the effective "return" value of a given call.

• Suture config should take a list of expected error types:

  Suture.create(:foo, {
    :old => ->{ raise MyErrorClass.new },
    :args => [],
    :expected_errors => [MyErrorClass, MyOtherError]
  })

• When recording the above, the "result" should be the error
• When verifying the above, if an error is rescued, check to see if it was expected and if it matches a permitted error type. Only mark the test result in error if it was not permitted.