SysTablespace::file_not_found(): If the system tablespace cannot be
found and innodb_force_recovery has been specified, refuse to start up.
The system tablespace is necessary for accessing any InnoDB tables,
because it contains the TRX_SYS page (the state of transactions)
and the InnoDB data dictionary.

This is similar to our handling of innodb_read_only except that
we will happily create the InnoDB temporary tablespace even if
innodb_force_recovry is set.