Add postgres.Set to handle PUT; small fixes

[synackd]

Addresses #55

This PR adds the Set() method in the postgres package and calls it from the Store() function in BSS when postgres is enabled. The Add() method in postgres is still used in StoreNew().

This PR also adds the following chores and refactorings:

• Typify postgres errors to be able to determine what type of error occurred
• Make error messages start with lowercase letter to be more idiomatic
• Fix phony target in Makefile to be container instead of docker
• Fix comment typos
• Fix incorrect 'PATCH' in 'PUT' log message