mxcl/homebrew — pull request #17422 by reinh (2013-01-30 00:44:18+00:00)
Title: MacOS::XQuartz#prefix raises better error if nil

A `nil` #prefix will currently cause an `undefined method '/' for nil:NilClass` error. This causes it to raise a more meaningful error.

TODO: tests, more idiomatic error class(?).