Try to make error message more obvious

Previously,

    Dependency on unbuildable library from darcs
    In the stanza 'test-suite darcs-test'
    In the inplace package 'darcs-2.11.0'

With this patch,

    Dependency on unbuildable (i.e. 'buildable: False') library from darcs
    In the stanza 'test-suite darcs-test'
    In the inplace package 'darcs-2.11.0'

Cabal/Distribution/Backpack/ConfiguredComponent.hs

@@ -169,7 +169,7 @@ toConfiguredComponent pkg_descr this_cid dep_map component = do
                     value <- case Map.lookup cn =<< Map.lookup pn dep_map of
                         Nothing ->
                             dieProgress $
-                                text "Dependency on unbuildable" <+>
+                                text "Dependency on unbuildable (i.e. 'buildable: False')" <+>
                                 text (showComponentName cn) <+>
                                 text "from" <+> disp pn
                         Just v -> return v