Problem was reporting wrong error

Fixed by adding a new error which better explain the problem.

mysql-test/r/partition_error.result:
  Bug#49161: Out of memory; restart server and try again (needed 2 bytes)
  
  Updated test result
mysql-test/t/partition_error.test:
  Bug#49161: Out of memory; restart server and try again (needed 2 bytes)
  
  Added test case
sql/ha_partition.cc:
  Bug#49161: Out of memory; restart server and try again (needed 2 bytes)
  
  Better error message. (used ER_UNKNOWN_ERROR to avoid merge
  problems in mysql-trunk+)