Add a detailed exit code flag to `convection diff`

[erran]

Description

Desired behaviour

A flag that we can set that causes a 2 exit status when we have a successful diff with differences.

Observed behaviour

A 0 exit status on successful diff with or without differences of the local and remote templates.

Motivation

This will allow us to remove noise from github pull request hooks as we do not need to comment when there are no differences found.