MDEV-18862 Unfortunate error message upon attempt to drop system versioning

Fix error code.

mysql-test/suite/versioning/r/alter.result

@@ -622,9 +622,9 @@ set sql_mode= default;
 set system_versioning_alter_history= keep;
 create or replace table t1 (x int) with system versioning;
 alter table t1 drop column `row_start`, drop column `row_end`, drop period for system_time, drop system versioning;
-ERROR HY000: No 'PERIOD FOR SYSTEM_TIME' in system-versioned `t1`
+ERROR 42000: Can't DROP PERIOD FOR SYSTEM_TIME on `t1`; check that it exists
 alter table t1 drop period for system_time;
-ERROR HY000: No 'PERIOD FOR SYSTEM_TIME' in system-versioned `t1`
+ERROR 42000: Can't DROP PERIOD FOR SYSTEM_TIME on `t1`; check that it exists
 alter table t1 drop column `row_start`, drop column `row_end`, drop system versioning;
 ERROR 42000: Can't DROP COLUMN `row_start`; check that it exists
 alter table t1 drop column `row_end`;

mysql-test/suite/versioning/t/alter.test

@@ -519,9 +519,9 @@ set sql_mode= default;
 --echo #
 set system_versioning_alter_history= keep;
 create or replace table t1 (x int) with system versioning;
---error ER_VERS_NO_PERIOD
+--error ER_CANT_DROP_FIELD_OR_KEY
 alter table t1 drop column `row_start`, drop column `row_end`, drop period for system_time, drop system versioning;
---error ER_VERS_NO_PERIOD
+--error ER_CANT_DROP_FIELD_OR_KEY
 alter table t1 drop period for system_time;
 --error ER_CANT_DROP_FIELD_OR_KEY
 alter table t1 drop column `row_start`, drop column `row_end`, drop system versioning;

sql/share/errmsg-utf8.txt

@@ -7898,5 +7898,3 @@ ER_KEY_DOESNT_SUPPORT
         eng "%s index %`s does not support this operation"
 ER_ALTER_OPERATION_TABLE_OPTIONS_NEED_REBUILD
 	eng "Changing table options requires the table to be rebuilt"
-ER_VERS_NO_PERIOD
-        eng "No 'PERIOD FOR SYSTEM_TIME' in system-versioned %`s"

sql/sql_table.cc

@@ -8078,7 +8078,7 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   }
   else if (alter_info->flags & ALTER_DROP_PERIOD && vers_system_invisible)
   {
-    my_error(ER_VERS_NO_PERIOD, MYF(0), table->s->table_name.str);
+    my_error(ER_CANT_DROP_FIELD_OR_KEY, MYF(0), "PERIOD FOR SYSTEM_TIME on", table->s->table_name.str);
     goto err;
   }
   alter_info->flags &= ~(ALTER_DROP_PERIOD | ALTER_ADD_PERIOD);