Don't blindly skip first migration.

The existing code would, no matter-what, skip the first migration when
running through the migrations tests. This makes the code
slightly counterintuitive (why skip the first one?), and is not very
flexible.

The reworked logic lets a migration raise a `NotImplementedError` to
signal that it doesn't support a `downgrade`. This lets the
walk-versions code treat the first-migration just like any other.

Change-Id: I19fd8ebc6075392cf9e573b8b4166291584b4a83
Merge "Don't blindly skip first migration."