Add a sentinel error `ErrNoActiveMigration` for the case where there is
no active migration. This improves the error strings presented to users
by not mentioning SQL errors.
**`pg-roll start` when there is a migration in progess:**
```
Error: a migration for schema "public" is already in progress
```
**`pg-roll rollback` when there is no migration in progress:**
```
Error: unable to get active migration: no active migration
```
**`pg-complete` when there is no active migration:**
```
Error: unable to get active migration: no active migration
```
This migrations introduces state handling by creating a dedicated
`pgroll` schema (name configurable). We will store migrations there, as
well as their state. So we keep some useful information, ie the
migration definition (so we don't need it for the `complete` state).
Schema includes the proper constraints to guarantee that:
* Only a migration is active at a time
* Migration history is linear (all migrations have a unique parent,
except the first one which is NULL)
* We now the current migration at all times
Some helper functions are included:
* `is_active_migration_period()` will return true if there is an active
migration.
* `latest_version()` will return the name of the latest version of the
schema.
