fix(throw on missing stats.json)

[dpchamps]

#problem

the loadAssetsFromStats function will return an empty object when stats.json cannot be found. This results in undefined behavior downstream: see the code path in #1558 as an example.
Should this fail -- whether from unexpected behavior or invalid inputs, the error delivered to the end-user is unrelated to the original failure.

#solution

Instead of preempting a file io error by returning an empty object, the function should throw with a helpful message. If consumers expect the file to be missing, the callsite should handle as necessary. Otherwise, the end-user get's an idea of what failed.

closes #1558

[CLAassistant]

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