allow a different webhook to be used to send messages on build failures

[weedmapsmike]

Checklist

• I've read and followed the Contribution Guidelines
• step.yml and README.md is updated with the changes (if needed)

Version

Requires a MAJOR/MINOR/PATCH version update

Context

When using a webhook to send slack messages, the channel in which the message is posted relies on the webhook's linked channel. This change allows a different channel to used to send messages to in the case of a failure, when webhooks are used and not an api token.

Changes

• add a new optional field in the yml for adding a webhook to use when the build fails.
• add a new property in the Config struct: WebhookURLOnError
• use the WebhookURLOnError when there is an error and it is populated

Investigation details

• this was the only solution I could think of to allow a different slack channel to be used when the build fails

Decisions

• tried to follow what already existed

[ofalvai]

LGTM, thank you for the contribution @weedmapsmike 🚀