Wrap read of cached training set metadata in try/except for robustness

[jeffreyftang]

In practice the read may sometimes fail even when the previous existence check passes, although whether this is due to a race condition or some quirk of the underlying filesystem isn't fully understood.

Converting an errored read into a cache miss seems reasonable and more robust behavior.

[github-actions]

Unit Test Results

       6 files  ±0         6 suites  ±0   2h 40m 2s ⏱️ - 3m 47s
2 966 tests ±0  2 917 ✔️ ±0    49 💤 ±0  0 ❌ ±0 
8 898 runs  ±0  8 715 ✔️ ±0  183 💤 ±0  0 ❌ ±0 

Results for commit 3ffc7ed. ± Comparison against base commit 6a9569a.

[justinxzhao]

Seems like a reasonable change to me!