Add nice dependency cycle error

[denisdefreyne]

Fixes #1111. This prints the first cycle, which should be enough information:

The site cannot be compiled because there is a dependency cycle:

    (1) item /b.md, rep :default, depends on
    (2) item /c.md, rep :default, depends on
    (3) item /d.md, rep :default, depends on
    (4) item /e.md, rep :default, depends on (1)