Bug #48248 assert in MDL_ticket::upgrade_shared_lock_to_exclusive

The assert would happen if REPAIR TABLE was used on a table already
locked by LOCK TABLES READ. REPAIR mistakenly tried to upgrade the
read-lock to exclusive, thereby triggering the assert.

The cause of the problem was that REPAIR TABLE ignored errors 
from opening and locking tables. This is by design, as REPAIR
can be used to broken tables that cannot be opened. However,
repair also ignored logical errors such as the inability to
exclusivly lock a table due to conflicting LOCK TABLES.

This patch fixes the problem by not ignoring errors from
opening and locking tables if inside LOCK TABLES mode.
In LOCK TABLES we already know that the table can be opened,
so that the failure to open must be a logical error.

Test added to repair.test.