Projections get stuck in a restart loop if an event is written to the checkpoint stream

[hayley-jean]

If an event is written to the projections checkpoint stream that does not have the $ProjectionCheckpoint type, the projection will get caught in a restart loop.

It does not error, and the UI makes it look like it's running successfully.
The projection can be stopped through the UI, but will get into the same restart loop if started again.

The projection should instead fault if the last event in the checkpoint stream is not a checkpoint event.

Reproduction steps:

1. Create a projection that has a low checkpoint count.
2. Write events so that the projection checkpoints.
3. Write an event to the projection checkpoint stream ($projections-{projection_name}-checkpoint) that is not a $ProjectionCheckpoint event.
4. Write more events so that the projection checkpoints again.
5. The projection will get into a restart loop.