Insurance — Automobile — Post-loss obligations — Examination under oath — Jury instructions that in order to prevail on its failure to cooperate defense, insurer was required to establish that insured did not comply with post-loss obligations and that insurer was “actually” prejudiced by this failure to comply was incorrect statement of law — To establish failure to cooperate defense, insurer must show “material failure” to cooperate which “substantially” prejudiced insurer — New trial required