Done, and also received feedback already! Here's the bug report: https://rt.cpan.org/Ticket/Display.html?id=39323

And here's the solution I was seeking, based on demo/demo_errors.pl:

use strict;
use Data::Dumper;
use Parse::RecDescent;

# Simple Lisp S-expressions (as an example)
my $grammar = q#
    start: expression(s?)
         | { die join("\n", map { $_->[0] } @{ $thisparser->{errors} }
+)."\n" }

    expression: /\w+/ 
              | '(' expression(s?) ')'
                { $item[2] }
              | <error>
#;

my $p = Parse::RecDescent->new($grammar);
# This will die if there is an error.
print Dumper($p->start('(foo (bar))'));

# Like so:
$p->start('(foo');
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