fix: dont wipe out snapshot if error in parser

[markphelps]

Fixes: #95

• Modifies the Snapshot::build to propagate any errors instead of returning an empty Document

We still swallow the error in replace_snapshot, and will need to figure out what we want to do here, likely something related to #60

This also returns an error on first call to new_snapshot_evaluator at engine initialization if it can't do the first call to get the snapshot from the API, which I think is likely the behavior we want for now

[yquansah]

Yeah i think this is fine for now. We will figure out logging, and other error scenarios later on.