Make stats.json not found error more helpful

[mikewilli]

We should make the "Unknown error: Could not find data in analysis bucket at path statistics/statistics_[...].json" message more helpful. We can change both the error type and message.

Error type could be something like:

• Results Not Available
• Results Fetch Error

Message could be something like:

• Results that were expected to be available could not be retrieved from the analysis bucket (path: statistics/statistics_[...].json. This likely indicates an error in analysis.

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