Add workaround for rails infected error classes

[mbj]

Fix exception serialization form rails infected code bases.
This case can happen when the killfork terminates abnormal,
and the resulting exception in the worker has to be propagated to
the main process for reporting.
On "normal" Ruby the exceptions are dump/loadable but rails and
its core extensions break this invariant. Hence mutant now
captures the essence of the exception in an attribute copy for
propagation.