Validate preconditions and show user-friendly messages

## Description

We have been actively utilizing the migration tool for shifting repositories from GHES to GHEC. The tool has been serving us well, and we are grateful for its convenience.

However, we often face complications arising from preparation mistakes (especially PATs). These issues often result in migration failure and require additional troubleshooting.

Currently, we are using a wrapper tool for the `gh gei` command to check pre-conditions as follows:

* The source organization and repository exists
* The source PAT has a valid set of OAuth scopes
* The source PAT has enough permission to generate migration archive in the organization
* The destination organization exists
* The destination repository DOES NOT exist
* The destination PAT has a valid set of OAuth scopes
* The destination PAT has enough permission to start migration to the organization

To streamline the process, it would be great if the tool runs the pre-condition check first before starting the migration process and provides a user-friendly error message when the check failed.