Error when a non-optional builder is not specified.
Error when a non-optional builder is not specified.

See #211.

sdiehl/ghc,snowleopard/hadrian,sdiehl/ghc,izgzhen/hadrian,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,bgamari/shaking-up-ghc,snowleopard/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc