Raise an error when missing (pending) migrations are detected

[mfridman]

The default state in goose is to ignore out-of-order migrations. If you subscribe to hybrid versioning approach then this will not affect you because out-of-order migrations aren't possible.

The intended behaviour for goose should be to detect older, unapplied, migrations and error if any are found.

To illustrate an example, suppose migration 1 and 4 are applied and then version 2 and 3 are introduced. Today goose will ignore version 2 and 3.

Database: goose_db_version

| version_id | is_applied |
|------------|------------|
|          0 | t          |
|          1 | t          |
|          4 | t          |

commands below abbreviated

$ goose status 
Applied At                  Migration
=======================================
Sat Oct 16 12:56:26 2021 -- 00001_a.sql
Pending                  -- 00002_b.sql
Pending                  -- 00003_c.sql
Sat Oct 16 12:56:26 2021 -- 00004_d.sql

$ goose up
goose: no migrations to run. current version: 4

$ goose up-by-one
goose: no migrations to run. current version: 4
goose run: no next version found

The fix is to return an error if pending migrations are detected. This will be the default.

After this PR is merged In a single PR I'll add support for out-of-order migrations as described #262. That way folks are not blocked and have a flag (or option) to apply migrations.

EDIT: I suppose there will be some people that are happy to ignore those pending migrations, not to be confused with the folks that would like to apply out-of-order migrations. For the former we could add a "disable validation" or something. But I'd like for someone to raise an issue for that use-case.

[mfridman]

Fixed by #280