[SHIPA-1384] skips notFound error in app watch loop

[stinkyfingers]

Description

When updating an existing app, especially via shipa app deploy, the app watcher loop occasionally emits a WARNING event, indicating that it cannot find the deployment intended to be deleted. It seems to be a race. It's probably fine to just skip this notFound error and let the loop run again.

I tested this PR by building & pushing this docker image, using it w/ shipa, and re-deploying an existing app to a new framework.

[shipa-1384]

From logging the error during an app update:

2022/03/29 18:27:13 Deleting "a1-web-9" in shipa-system...
ERR <nil> false
2022/03/29 18:27:13 updating status for upgraded release for a1
ERR deployments.apps "a1-web-9" not found true

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Chore (documentation addition or typo, file relocation)

Testing

• New tests were added with this PR that prove my fix is effective or that my feature works (describe below this bullet)
• This change requires no testing (i.e. documentation update)

Documentation

• All added public packages, funcs, and types have been documented with doc comments
• I have commented my code, particularly in hard-to-understand areas

Final Checklist:

• I followed standard GitHub flow guidelines
• I have performed a self-review of my own code
• My changes generate no new warnings