FailureMarker should be responsible for excluding the marker file from stage operations

Problem/Motivation

Right now, we have explicit logic in StageBase to exclude the failure marker file from stage operations (in particular, the apply phase). I would argue that this is not awesome for two reasons:

1. The FailureMarker class should be fully responsible for managing the marker file.
2. It should be excluded from all stage operations, not just the apply.

Proposed resolution

Make FailureMarker an event subscriber that listens to CollectPathsToExcludeEvent, and adds the failure marker file as an excluded path. This will also simplify StageBase::apply() a tad.

Remaining tasks

Do it and add explicit test coverage.