handler::get_dup_key used the called handler for the
info call, but used table->file handler for errkey.
Fixed by using table->file->info instead.

mysql-test/r/partition_error.result:
  Bug#38719: Partitioning returns a different error code for
  a duplicate key error
  
  Added test for verification
mysql-test/t/partition_error.test:
  Bug#38719: Partitioning returns a different error code for
  a duplicate key error
  
  Added test for verification