Clarify access to a non-existent `Object` member

[jdidion]

Currently the spec does not state whether accessing an object member that does not exist results in a None or an error. E.g.

Object foo = object { a: 1 }
Int b = foo.b  # what happens here??

I suggest that the result should be a None, since there is currently no way to check the existence of an Object member.

[rhpvorderman]

I suggest an error.

input:
  my_optional_string=foo.b

Should throw an error when b does not exist, rather than silently putting in a None.

[jdidion]

That's a good point - I would expect an error in that case.

Maybe we can have contains_key() (#305 ) accept either a Map or Object?

[patmagee]

I am in favour of an error as well with a preference for using the contains_key function. Most languages raise some sort of access error when you are trying to do a direct access and the key does not exist