as with impure derivations it is still possible to garbage-collect
existing xp-dyn-drv derivations. we once again don't introduce any
new kinds of errors, we only change the dynamic type of exceptions
from MissingExperimentalFeature to UnimplementedError (although we
do throw FormatError when reading xp-dyn-drv derivations now, that
seems to make a little more sense than "feature not implemented").
Change-Id: Ic26e5b6c9c9e2533093e27f6cf901dc9db57c83e