fix acorn run --update behavior to allow --name flag (#1444 #1457)

[jsilverio22]

Checklist

• The title of this PR would make a good line in Acorn's Release Note's Changelog
• The title of this PR ends with a link to the main issue being address in parentheses, like: This is a title (#1216). Here's an example
• All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
  acorn run --update will not create if --name is set  #1444 Need more helpful error message in cases when IMAGE|DIRECTORY or appname is missing. #1457
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• Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
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This PR changes the behavior when a user does acorn run --update -n app_name prior if the app did not exist an error would be thrown, after the behavior will be that a new app w/ name app_name will be created

[jacobdonenfeld]

Is allowing --name flag with --update covered in tests?

[jsilverio22]

Is allowing --name flag with --update covered in tests?

from the hub e2e I believe so