Add a utility that requires the existence of a given executable (like ffmpeg) on PATH. Possibly just sugar for a general "abort unless this block evals true" utility.
# Oops! I need `ffmpeg`, and it isn't in your PATH.
need :ffmpeg, "to transcode audio"
# Oops! I need `ffmpeg` to transcode audio, and it isn't in your PATH.
need "MongoDB", :executable => "mongod"
# Oops! I need MongoDB, and `mongod` isn't in your PATH.
need "GIS files", :path => "/opt/gis-files"
# Oops! I need GIS files, and '/opt/gis-files' doesn't exist.
Think about this after 2.0.