url: .title command shows an error for links handled by another plugin

[dgw]

Description

Links handled by another plugin (for example links to GitHub issues, if the github plugin is installed) are skipped intentionally in the .title command, to avoid giving duplicate output. However, the error message code from #2029 can't tell the difference between a link that was intentionally skipped and a link for which a title couldn't be fetched.

Reproduction steps

1. Command Sopel, .title http://some.link/that/another/plugin/handles
   • With core plugins only, a Reddit post is a good way to trigger this.
2. url responds with an error: "Sorry, fetching that title failed. Make sure the site is working."
   AND the plugin that knows how to handle the link will also reply

Expected behavior

The url plugin should be silent in this case, unless multiple URLs are passed to .title and at least one of them isn't handled by another plugin.

Environment

• Sopel .version: 7.1.0-dev
• Sopel installed via: pip install -e
• Python version: 3.8.5
• Operating system: Ubuntu 18.04
• Relevant plugins: url

Notes

Likely easiest to fix this in conjunction with #2054. I'm therefore marking this for 8.0 even though it would be nice to fix it in 7.1. (The target milestone is, as always, subject to change after discussion.)