Add more descriptive error messages

[Atreya Shankar]

🚀 Feature Request

Checklist

• No duplicate issues/PRs
• Are you running the latest downgrade release from the AUR?

Background

As mentioned in #168 (comment), downgrade only provides an uninformative error message Unable to downgrade <pkg>.

Proposed feature

We should provide more descriptive error messages, such as no matching packages found, no results found, etc.