Only add Error_no to event if there is one. Don't stringify it either; set its type in pqd.

bin/pt-query-digest

@@ -14591,6 +14591,7 @@ sub main {
          Last_errno    => 'string',
          Thread_id     => 'string',
          InnoDB_trx_id => 'string',
+         Error_no      => 'string',
          Killed        => 'bool',
       };
 

lib/MySQLProtocolParser.pm

@@ -654,11 +654,14 @@ sub _packet_from_server {
             }
 
             $event = {
-               cmd       => $com,
-               arg       => $arg,
-               ts        => $packet->{ts},
-               Error_no  => $error->{errno} ? "#$error->{errno}" : 'none',
+               cmd => $com,
+               arg => $arg,
+               ts  => $packet->{ts},
             };
+            if ( $error->{errno} ) {
+               # https://bugs.launchpad.net/percona-toolkit/+bug/823411
+               $event->{Error_no} = $error->{errno};
+            }
             $session->{attribs}->{Error_msg} = $error->{message};
             return $self->_make_event($event, $packet, $session);
          }

t/lib/MySQLProtocolParser.t

@@ -177,7 +177,7 @@ test_protocol_parser(
          pos_in_log => 0,
          bytes      => length('select 5 from foo'),
          cmd        => 'Query',
-         Error_no   => "#1046",
+         Error_no   => "1046",
          Error_msg  => 'No database selected',
          Rows_affected => 0,
          Warning_count      => 0,
@@ -1659,7 +1659,7 @@ test_protocol_parser(
    [
       {
          Error_msg          => "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1",
-         Error_no           => '#1064',
+         Error_no           => '1064',
          No_good_index_used => 'No',
          No_index_used      => 'No',
          Query_time         => '0.000316',
@@ -1679,7 +1679,7 @@ test_protocol_parser(
       },
       {
          Error_msg          => 'Unknown system variable \'nono\'',
-         Error_no           => '#1193',
+         Error_no           => '1193',
          No_good_index_used => 'No',
          No_index_used      => 'No',
          Query_time         => '0.000329',