Improve handling of missing variables

[prkunz]

The current behavior is typically a segfault :face_palm:

There are two things we can do better:

1. Detect the missing variable response when unpacking messages from sim: "The message returned for the non-existent variable will have a type of 24 and it's value will be the string 'BAD_REF'."
2. Ask Trick if variables exist using the var_exists command

I think we'll want to do both, since the latter will be the only way to check if variables used in commands exist.

When a variable is missing, the easiest response is to exit the app.

However we could do better and just turn off individual messages.