Move Object Store to using std::io::Error

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The ObjectStore crate currently makes use of Snafu to provide structured errors, these are then mapped to crate-level error type. This plumbing is confusing, verbose, adds limited additional utility, and leads to unnecessary breaking changes when adding new variants.

Describe the solution you'd like

The standard library has already developed an error abstraction for IO, namely std::io::Error. We should just use this

Describe alternatives you've considered

We could not do this

Additional context

[tustvold]

Closing in favour of #2725