PendingMigrationsError: Change to more generic DatabaseError

[mao-sz]

Because

Currently, one of the startup checks is whether there are pending migrations to apply. This isn't the only database-related startup error though. Right now, if the connection string isn't accurate or postgresql.service isn't running, it still reports the pending migrations error.
dbmate status --exit-code returns 1 when connected but pending migrations, and 2 for db connection issues.

I do not know if postgresql.service and systemctl is different for different distros, or how it's handled on macOS (macOS guide doesn't mention anything about it), so I've left that error message more general for people to google if they run into it.

This PR

• Renames PendingMigrationsError to the more generic DatabaseError
• Ensures a relevant error message depending on the actual cause (both for the bot and for tests)

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. Callbacks command: Update verbiage
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR adds new features or functionality, I have added new tests
• If applicable, I have ensured all tests related to any command files included in this PR pass, and/or all snapshots are up to date

[mao-sz]

When postgresql.service isn't running:

When service is running and connection string correct but pending migrations:

When postgresql.service isn't running and connection string wrong (when running Jest):