`TypeError: err.toJSON is not a function` when webhook URL is invalid

This took a bit for me to track down (my own mistake), thanks to the real error not being shown in the action. If, for example, I accidentally quote the webhook URL, like:

           - name: Notify Slack
              uses: slackapi/slack-github-action@v2
              with:
                  errors: true
                  payload: <redacted>
                  webhook: ${{ toJSON(secrets.SLACK_WEBHOOK_URL) }}
                  webhook-type: webhook-trigger

I get an exception thrown:

Run slackapi/slack-github-action@v2
Error: TypeError: err.toJSON is not a function

Error: err.toJSON is not a function
TypeError: err.toJSON is not a function
    at Webhook.post (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/webhook.js:36:1)
    at post (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/send.js:35:1)
    at send (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/send.js:15:1)
    at file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/index.js:9:1
file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/webhook.js:36
      const response = err.toJSON();
^
TypeError: err.toJSON is not a function
    at Webhook.post (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/webhook.js:36:1)
    at post (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/send.js:35:1)
    at send (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/send.js:15:1)
    at file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/index.js:9:1

It looks like the catch block is assuming the exception will always have a toJSON function, but in my case it obviously doesn't.

The fix of course, was to remove the erroneous toJSON(...) wrapper from the webhook input, but it would have saved me a lot of time if the exception was handled better by thi action.

Note, the err.toJSON in the exception is unrelated to the toJSON in my input - its only coincidental that we both used (different) toJSON functions (JavaScript vs GitHub Expression respectively).

Cheers.

Start in src/webhook.js at the catch block reported at line 36, then review how it is called from src/send.js and src/index.js. Ensure an invalid webhook URL reports the underlying error without assuming the exception has toJSON; done means the original failure is shown instead of a secondary TypeError.