SHOW CREATE TABLE fails

Underlying table names, that merge engine fails to open were not
reported.

With this fix CHECK TABLE issued against merge table reports all
underlying table names that it fails to open. Other statements
are unaffected, that is underlying table names are not included
into error message.

This fix doesn't solve SHOW CREATE TABLE issue.


myisammrg/myrg_def.h:
  Added myrg_print_wrong_table declaration.
myisammrg/myrg_open.c:
  If HA_OPEN_FOR_REPAIR is passed to merge engine open function,
  report names of tables that we fail to open.
mysql-test/r/backup.result:
  Updated test result.
mysql-test/r/key_cache.result:
  Updated test result - removed duplicate error.
mysql-test/r/lock.result:
  Updated test result - added summary row.
mysql-test/r/merge.result:
  A test case for BUG#26976.
mysql-test/r/preload.result:
  Updated test result - removed duplicate error, added summary row.
mysql-test/r/ps.result:
  Updated test result - removed duplicate error, added summary row.
mysql-test/r/repair.result:
  Updated test result - removed duplicate error, added summary row.
mysql-test/r/rpl_failed_optimize.result:
  Updated test result - removed duplicate error, added summary row.
mysql-test/r/sp.result:
  Updated test result - removed duplicate error, added summary row.
mysql-test/r/view.result:
  Updated test result - removed duplicate error, added summary row.
mysql-test/t/merge.test:
  A test case for BUG#26976.
sql/ha_myisam.cc:
  Do not report same error twice.
sql/ha_myisammrg.cc:
  If HA_OPEN_FOR_REPAIR is passed to merge engine open function,
  report names of tables that we fail to open.
  
  Added dummy ha_myisammrg::check to not confuse users with
  "not implemented" error in case all underlying tables are
  fine.
sql/ha_myisammrg.h:
   Added ha_myisammrg::check declaration.
sql/share/errmsg.txt:
  Added ER_ADMIN_WRONG_MRG_TABLE errno. It is used instead
  of ER_WRONG_MRG_TABLE in case HA_OPEN_FOR_REPAIR is
  passed to merge engine handler open function.
sql/sql_error.cc:
  warning_level_* are now public. It is required by mysql_admin_table to
  report message level.
sql/sql_error.h:
  warning_level_* are now public. It is required by mysql_admin_table to
  report message level.
sql/sql_table.cc:
  Reorder mysql_admin_table arguments to meet it's definition.
  
  Report errors that are pending in thd->warn_list as results of
  admin function.