DUX-3102: expose more validation error content

[lf-]

Slack's API can return something like:

{
    "ok": true,
    "warnings": [
        "missing_charset"
    ],
    "response_metadata": {
        "warnings": [
            "missing_charset"
        ],
        "messages": [
            "[WARN] A Content-Type HTTP header was presented but did not declare a charset, such as a 'utf-8'"
        ]
    }
}

Source: https://api.slack.com/changelog/2016-09-28-response-metadata-is-on-the-way

This is helpful because they secretly use json schema on their end to validate what is sent to them but don't publish the schema (grrr), and their errors are actually quite helpful. Previously slack-web did not expose them at all.

This is a breaking change, unfortunately, but it should at least improve debugging greatly.

Before submitting your PR, check that you've:

• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Bumped the version number if there isn't an (unreleased) on the Changelog
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[linear]

DUX-3102 Slack-web loses context of JSON validation error responses