logging: add informative logging in case mounted pacta-data contains none of the expected data

[jdhoffa]

Currently, if the pacta-data directory is volume mounted incorrectly, the error message points to an issue with currencies.rds (since this is the first file that is read).
We could improve the developer experience by doing a first check if there are even any .rds files in the mounted directory at all. If not, give an informative error.

Current behaviour

cannot open compressed file
'/mnt/pacta-data/2023Q4_20240718T150252Z/currencies.rds', probable
reason 'No such file or directory'

Desired behaviour

Something along the lines of (feel free to adapt):

Error: No `.rds` files found in the specified `pacta-data` directory. 
Please ensure the volume mount is configured correctly and that the 
directory contains the expected files.

Hint: If you believe the files are present, check that the file paths match 
exactly, and verify the volume mount configuration in your setup.

AB#12256