[Functions] Response body after Timeout is mangled and non-informative

[caesarsol]

Describe the bug

The response of a Netlify Function incurring in timeout is �[91m◈�[39m Function invocation failed: [object Object] instead of a clear explanation of the timeout issue.

To Reproduce

Steps to reproduce the behavior:

1. Create a function taking >10s to perform:

   async function handler(event, context, callback) {
     await new Promise((r) => setTimeout(r, 20_000))
     return callback(null, {})
   }

2. Run netlify dev
3. Call the associated endpoint
4. Wait 10s (the default timeout)
5. Response will be non-informative and malformatted:

   HTTP/1.1 500 Internal Server Error
   
   �[91m◈�[39m Function invocation failed: [object Object]
   

Configuration

• netlify.toml:

   [build]
     functions = "./functions"
   
   [dev]
     framework = "create-react-app"
  

• npx envinfo:

     System:
       OS: macOS 10.15.7
       CPU: (8) x64 Intel(R) Core(TM) i7-8559U CPU @ 2.70GHz
       Memory: 279.05 MB / 16.00 GB
       Shell: 5.7.1 - /bin/zsh
     Binaries:
       Node: 12.16.1 - ~/.nvm/versions/node/v12.16.1/bin/node
       Yarn: 1.16.0 - ~/.yarn/bin/yarn
       npm: 6.13.4 - ~/.nvm/versions/node/v12.16.1/bin/npm
  

Expected behavior

A more informative output.
Maybe in JSON format, or customizable by the user?

CLI Output

◈ Error during invocation: {
  errorMessage: 'Task timed out after 10.00 seconds',
  errorType: 'TimeoutError',
  stackTrace: [
    'new TimeoutError (.../node_modules/netlify-cli/node_modules/lambda-local/build/lib/utils.js:110:28)',
    'Context.<anonymous> (.../node_modules/netlify-cli/node_modules/lambda-local/build/lib/context.js:110:19)',
    'listOnTimeout (internal/timers.js:549:17)',
    'processTimers (internal/timers.js:492:7)'
  ],
  level: 'error',
  [Symbol(level)]: 'error',
  [Symbol(message)]: '{"errorMessage":"Task timed out after 10.00 seconds","errorType":"TimeoutError","stackTrace":["new TimeoutError (.../node_modules/netlify-cli/node_modules/lambda-local/build/lib/utils.js:110:28)","Context.<anonymous> (.../node_modules/netlify-cli/node_modules/lambda-local/build/lib/context.js:110:19)","listOnTimeout (internal/timers.js:549:17)","processTimers (internal/timers.js:492:7)"],"level":"error"}'
}

Additional context

This is what I've found:

• the message Function invocation failed is created here:
  

  cli/src/utils/serve-functions.js

  Lines 20 to 25 in f1b2502

  	function handleErr(err, response) {
  	response.statusCode = 500
  	response.write(`${NETLIFYDEVERR} Function invocation failed: ` + err.toString())
  	response.end()
  	console.log(`${NETLIFYDEVERR} Error during invocation:`, err)
  	}

• it's prepended with a terminal-colored ◈ char for console output, but being not used in console this messes up the output with non utf-8 chars
• the [object Object] is the result of an error.toString() on something which apparently inherits from Object
• the TimeoutError is generated by lambda-local, but gets later translated into a plain Object here:
  https://github.com/ashiina/lambda-local/blob/develop/src/lib/context.ts#L181-L185

I am now going to write a quick PR to make the actual error appear in the HTTP body.

[caesarsol]

PR sent, let me know if I can further help.