If you have a proxy in place the best path forward would be to make this customization via the proxy server you're using due to the fact that the changes in BitBucket itself will revert after an upgrade, etc. Here are some resources you can use to create error pages in the environment you're using as a Proxy:
Bitbucket Pipelines helps me manage and automate a number of serverless deployments to AWS Lambda and this is how I do it. I'm building Node.js Lambda functions using node-lambda ...
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