Improved error message and issue template for missing rule mergers

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing issue: fixes Better error reporting for "no merger" complaints #143
• That issue was marked as status: accepting prs

Overview

Switched the error message printed to the error file to be more brief.

Error: multiple output some-rule ESLint rule options were generated, but tslint-to-eslint-config doesn't have "merger" logic to deal with this.
Please file an issue at https://github.com/typescript-eslint/tslint-to-eslint-config/issues/new?template=missing_merger.md 🙏

Also adds a corresponding issue template for this specific case.

FYI @joeyj-msft 😊

[JoshuaKGoldberg]

Published in 0.2.5! ✨