Add Cymbal branding to error page

[NimJay]

Background

• When enabling Cymbal branding (e.g., via https://github.com/GoogleCloudPlatform/microservices-demo/tree/release/v0.4.0/kustomize/components/cymbal-branding), the Cymbal logo was not being displayed in the app's "error" page.

Fixes

#1166

Change Summary

• Add the is_cymbal_brand/isCymbalBrand variable to the template that renders the error page.

Testing Procedure

• If you would like to be thorough, you can
  1. skaffold dev this branch (nimjay/cymbal-error)
  2. Set the CYMBAL_BRANDING to "true" inside /kubernetes-manifests/frontend.yaml.
  3. kubectl delete deployment/cartservice to bring up the error page.
• But I have done this testing myself and it worked! So feel free to skip testing during your review. :)

[github-actions]

🚲 PR staged at http://34.171.122.31

[NimJay]

Update: I have tested this myself and it works!

See the "Cymbal" logo in the error page:

[github-actions]

🚲 PR staged at http://34.171.122.31

[bourgeoisor]

LGTM