fix(cli): Improve error output message when model/checkpoint is not provided in finetune create

[blainekasten]

Before:

$ together fine-tuning create --training-file file-123-4324-2342
Usage: together fine-tuning create [OPTIONS]
Try 'together fine-tuning create --help' for help.
 
Error: Invalid value: You must specify either a model or a checkpoint

After:

$ together fine-tuning create --training-file file-123-4324-2342
Usage: together fine-tuning create [OPTIONS]
Try 'together fine-tuning create --help' for help.

Error: Missing option --model or --from-checkpoint.