Stop on first error during `get` and `exist` in `group.rs`

[ikopylov]

Right now errors are skipped. It is better to stop on error to prevent returning of old and incorrect version of data.

https://github.com/qoollo/bob/blob/master/bob-backend/src/pearl/group.rs#L273 https://github.com/qoollo/bob/blob/master/bob-backend/src/pearl/group.rs#L314