Improve the error message when a parent model folder is missing. (Translate)

When a parent model folder is missing then the config file for that parent folder cannot be found. That leads to an error like this:

FileNotFoundError: [Errno 2] No such file or directory: '/home/david/gutenberg/MT/experiments/BT-Swahili/en-swh-8/config.yml'

I'm running a child model where the folder name begins en-swh-8 It took me a while to realise that there wasn't a problem with finding the child model config file but that the whole parent model folder was missing.

A few improvements would be:

1. Indicate to the user that the child model folder and config file exist.
2. If a file is missing indicate which parts of the path are missing.
3. If we could indicate that 'line x of the config file refers to file which can't be found.' That would be very helpful.
4. Prior to preprocessing we check that the required files exist before trying to preprocess.
   Ensure that similar checks occur (those that are required) for each part of the pipeline.

Start by reproducing the missing parent model-folder case using the child model path and config.yml example in the issue. Trace the preprocessing and other pipeline stages to identify where required-file checks occur; done should include clearer missing-path context and checks for required files throughout the relevant pipeline.