sub cgiapp_prerun {
  my $this = shift;
  my $rm   = shift;

  eval {
    ...
  };
  if($@) {
    $this->stash->{exception} = "Error in prerun: $@";
    $this->prerun_mode("error");
  }
}

sub error {     # error-runmode
  my $this = shift;
  my $exception = shift        || 
     $this->stash->{exception} ||
     "default error message goes here";

 ... 
}