Improve scan error message for bad SLACK_URL

[mfurtak]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Description

Improve error messaging when scan's SlackPoster gets a 404 back from Slack

Motivation and Context

When a SLACK_URL is given, but it is not quite right, people were seeing this error message in their scan run logs. It's rather impossible to know what to do with this message.

[16:11:13]: ▸ Test Succeeded
[16:11:13]: #<Net::HTTPNotFound:0x007f93332e8df8>

This PR changes the error message in this case to be:

[16:15:18]: ▸ Test Succeeded
[16:15:19]: The Slack URL you provided (https://hooks.slack.com/services/XXX/XXX/XXX) could not be reached (404)

[hjanuschka]

such a great find 💯

[KrauseFx]

This is beautiful 👍 Nice find