Action fails silently if SLACK_BOT_TOKEN is not set

[gr2m]

Description

Action fails silently if SLACK_BOT_TOKEN is not set

What type of issue is this? (place an x in one of the [ ])

• [x] bug
• [ ] enhancement (feature request)
• [ ] question
• [ ] documentation related
• [ ] example code related
• [ ] testing related
• [ ] discussion

Requirements (place an x in each of the [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

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Bug Report

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Reproducible in:

package version: 1.15.0

node version: I don't think you should ask that, as it's configured in your own action.yml

OS version(s): ubuntu-latest

Steps to reproduce:

        env:
          SLACK_BOT_TOKEN: ${{ secrets.DOES_NOT_EXIST }}

Expected result:

Should error with a helpful message

Actual result:

No output, success status

Attachments:

[seratch]

Thanks again. As with https://github.com/slackapi/slack-github-action/issues/9, the error can be improved by having some validation as part of the action.