MDEV-26710: Histogram field in mysql.column_stats is too short

Change it to LONGBLOB.
Also, update_statistics_for_table() should not "swallow" an error
from open_stat_tables.

scripts/mysql_system_tables_fix.sql

@@ -842,4 +842,4 @@ DELIMITER ;
 
 ALTER TABLE column_stats
   modify hist_type enum('SINGLE_PREC_HB','DOUBLE_PREC_HB','JSON_HB'),
-  modify histogram blob;
+  modify histogram longblob;