Unset ncdata namelist variable causes crash

Currently, if `ncdata = UNSET_PATH`, the model crashes without a helpful message. While it would be great to catch this before a job is submitted (do we always need an `ncdata` file?), at runtime, this should be caught by calling `cam_get_file` with the proper arguments to return a failure code so `cam_initfiles_open` can call `endrun` with a helpful message.