Fetching a Property from an unknown name should result in an error

Summary: Currently, you just silently get None if you fetch a Property off of an unknown name in QL. But it's most or less always a mistake (most often because the name is a typo), so it should be an error.

That is, we're trying to guard against cases like this:

My Ting -> My Property

In this case, My Ting is a typo for My Thing, and it is Really Annoying that this doesn't result in an error.

We are, however, intentionally continuing to allow you to dereference a Tag Type value silently, so that you can have a mix of defined and undefined Tag values in a Property. Yes, that's a similar situation, but I think it's a bit less suspicious. We might review this decision later. (The critical code is in Invocation.bundlesAndContextsForProp.withCurrentContext().)