Handle corrupted prompts at load time

[joerunde]

Describe the bug

When caikit uses load_prompt_artifacts to copy over a prompt model's artifacts, there is no error handling if the files themselves are corrupted, so the load will succeed from caikit's perspective, but the model will be unusable in TGIS.

Sample Code

This is a logic bug, so we've never actually made it happen, but it could!

Expected behavior

If the prompt artifacts are corrupted and result in an unusable prompt in TGIS, an error should be reported so that the caikit load function can error out correctly.