Make naif_keywords explictly fail when key does not exist

[Kelvinrr]

It seems that when a NAIF keyword isn't in the naif_keyword dictionary, ALE stores a null instead of throwing. This probably has to do with how the props are being evaluated.

  "focal_length_model": {
    "focal_length": null
  },
  "detector_center": {
    "line": null,
    "sample": null
  },
  "focal2pixel_lines": null,
  "focal2pixel_samples": null

This causes CSM creation to fail and for users to dig through the logs to uncover the issue. It would be better to fail at ISD creation.