Changed the error reporting (and a crash) when inserting data into a
 MERGE table that has no underlying tables or no INSERT_METHOD specified
 by reporting that it is read-only.


include/my_base.h:
  Add new handler error
mysql-test/r/merge.result:
  Update results
mysql-test/t/merge.test:
  Add new regression test
sql/ha_myisammrg.cc:
  When trying to insert into a MERGE table with no underlying tables
  or no INSERT_METHOD, report that it is read-only.
sql/handler.cc:
  Handle new error message