Exit with Correct Status

[patrick-ogrady]

Motivation

Fix #20

Solution

• Add upstream change to make sure errors are returned correctly (Return Correct Error on Context Cancellation mesh-sdk-go#30).
• Check if error returned by errgroup is just context.Canceled (error status 0) or something else (error status 1).

Other Changes

• Remove NewAccountSeen as it is no longer part of the reconciler.Handler interface.