Add function to check for pending migrations
Add function to check for pending migrations

This change introduces the function any_pending_migrations, which
returns a `Result` wrapping a `bool`, which will be true if there are
any pending migrations, and false if there are none. `Err` states
indicate problems with the migration setup.

Fixes #811

sgrif/diesel,diesel-rs/diesel,sgrif/diesel,diesel-rs/diesel