Warn when referencing nonexistent Track()

[apotonick]

Given that the :skip track doesn't exist, this will be wired to :success.

step :_destroy?,
              Output(:success) => Track(:skip)

We need the ability to reference a track before it "got created" due to the way Ruby and DSLs (usually) work. However, if we only somehow could warn the user they're referencing a track that's nonexistent, that'd be cool. I wouldn't know how, though, unless we switch to a block-style DSL where we know when exactly a DSL use is finished and can "finalize" the data structure created from it.

[apotonick]

This would also work with Subprocess(Validate, strict: true): if a unknown track from Validate is referenced, this should break.

Problem is, we never know when we're finished collecting steps through the DSL.