This assertion could be triggered during execution of OPTIMIZE TABLE for
InnoDB tables. As part of optimize for InnoDB tables, the table is recreated
and then opened again. If the reopen failed for any reason, the assertion
would be triggered. This could for example be caused by a concurrent DROP
TABLE executed by a different connection. The reason for the assertion was
that any failures during reopening were ignored.

This patch fixes the problem by making sure that the result of reopening the
table is checked and that any error messages are sent to the client.

Test case added to innodb_mysql_sync.test.