Implement logic for Custom Error when contributor does not have OpenSearch-bot Installed

[BigSamu]

Refactoring in issue #32 is in progress. After merging the new code, a logic needs to be implemented for a Custom Error when a contributor does not have the OpenSearch-bot Installed. A post message should be sent to the PR conversation indicating that the bot is not installed and the contributor has 2 options to create the changeset:

• Manually incorporating a pr_number.yml file in the folder changelog/fragments
• Installing the bot using the following URL: https://github.com/apps/opensearch-bot