update-fingerprints: pretty print + sort json

[derekperkins]

As it stands today, the weekly commit updating fingerprints_data.json was all on a single line, with randomly sorted keys, making it near impossible to diff. By sorting and using MarshalIndent, it will be much easier to see what changes happen each week and to do reasonable git blame.

As for the implementation, jsoniter wasn't indenting properly, and didn't otherwise seem to provide any advantages over the stdlib, so I replaced it. Additionally, I added error checks that were being ignored before.

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