Handle vessel destruction errors

[rallen10]

Currently, if vessel's collide and one is destroyed (e.g. the Guard), errors are thrown during episode termination because calls to krpc reference a vessel that no longer exists

[rallen10]

This needs to be handled for all vessels (Lady, Bandit, Guard)

[rallen10]

This is a tricky one because I think it requires finding everywhere in the code that calls something like vesLady, vesBandit, or vesGuard and writing logic to handle the case when those vehicles don't exist. These calls are ubiquitous (fortunately, I think they can only be made within the ksp_interface_loop process, so at least they shouldn't be spread across processes which should make the logic a bit easier).

Instead of implementing this error handling logic at every call to something like vesGuard, it should obviously be implemented in one centralized place; e.g. create classes for these vessels and use methods to access their information while implementing the non-existence case-handling within these methods. However, this would still likely involve updating the call signatures for vesGuard, etc. everywhere they appear. Even if the call signatures could be left as-is, this change would require a large amount of testing that I don't have time for at the moment