CLI cannot handle non Error-typed thrown errors

[tdbit]

Describe the bug

The CLI will crash if a function throws a non Error type. It is good practice to always throw an Error type but it is not always guaranteed. Developers might throw custom error types, their own objects, or even (in our case) a simple string. This is perhaps not great engineering but it happens. When that non-standard error type is thrown inside a Netlify function it crashes the CLI as follows:

TypeError: Cannot read properties of undefined (reading 'map')
    at getNormalizedError (file:///Users/tdbit/.nvm/versions/node/v20.9.0/lib/node_modules/netlify-cli/dist/lib/functions/synchronous.js:88:41)
    at handleSynchronousFunction (file:///Users/tdbit/.nvm/versions/node/v20.9.0/lib/node_modules/netlify-cli/dist/lib/functions/synchronous.js:33:23)
    at handler (file:///Users/tdbit/.nvm/versions/node/v20.9.0/lib/node_modules/netlify-cli/dist/lib/functions/server.js:203:13)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)

  System:
    OS: macOS 14.5
    CPU: (10) arm64 Apple M1 Max
  Binaries:
    Node: 20.9.0 - ~/.nvm/versions/node/v20.9.0/bin/node
    Yarn: 1.22.22 - /opt/homebrew/bin/yarn
    npm: 10.1.0 - ~/.nvm/versions/node/v20.9.0/bin/npm
  Browsers:
    Chrome: 126.0.6478.127
    Safari: 17.5
  npmGlobalPackages:
    netlify-cli: 17.33.0

This is because getNormalizedError assumes the error will have a .stackTrace property.

Steps to reproduce

1. Add new Netlify function baderror.js to project:

export async function handler(event, context) {
throw "What?"
}

2. Run netlify dev
3. Hit localhost:8080/.netlify/functions/baderror

Configuration

No response

Environment

System:
OS: macOS 14.5
CPU: (10) arm64 Apple M1 Max
Memory: 5.24 GB / 64.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 20.9.0 - ~/.nvm/versions/node/v20.9.0/bin/node
Yarn: 1.22.22 - /opt/homebrew/bin/yarn
npm: 10.1.0 - ~/.nvm/versions/node/v20.9.0/bin/npm
npmGlobalPackages:
netlify-cli: 17.33.0