message: Add exception translation and logging to controller_impl.
diff: @@ -130,8 +130,10 @@ struct controller_impl {
  void emit( const Signal& s, Arg&& a ) {
  try {
  s(std::forward<Arg>(a));
  + } catch ( fc::exception& e ) {
  + wlog( "${details}", ("details", e.to_detail_string()) );
  } catch ( ... ) {
  - elog( "signal handler threw exception" );
  + wlog( "signal handler threw exception" );
  }
  }
  