Raise an exception if cache limit is too low

[karan6181]

Description of changes:

• If cache_limit is lower than a shard file size or lower than a few shard files sizes, raise an exception to raise a cache_limit. This is more of a better user experience. Hard to see a practical use-case where the cache_limit is this much low.

Issue #, if available:

#417

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[snarayan21]

other than the logic for getting the max shard size, lgtm!

[snarayan21]

lgtm!