Handle non-atomic updates, where the latest folder of NWP data might be incomplete

[JackKelly]

Many thanks to @emfdavid for pointing this out!

The issue is that, when reading from the most recent folder of NWP data, some files might be missing, and some files might be corrupt / incomplete (because the files are still being uploaded by the NWP provider).

hypergrib will need to handle this without falling over.

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