[services] refactor subroutines to use new error types

Summary:
we can now use the DBItemError type in the subroutines that handle parsing/unpacking DynamoDB attributes

Depends on D4427

Test Plan: printed out some of the errors to see how they're logged. cargo build + cargo run server

Reviewers: palys-swm, jimpo, karol-bisztyga

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4428