It was hard to understand what the error really meant.

The error checking in partitioning is done in several different
parts during the execution of a query which can make it
hard to return useful errors.

Added a new error for bad VALUES part in the per PARTITION clause.
Using the more verbose error that a column is not allowed in
the partitioning function instead of just that the function is
not allowed.

mysql-test/r/partition.result:
  changed error to be more specific
mysql-test/r/partition_error.result:
  updated result
mysql-test/std_data/parts/t1TIMESTAMP.frm:
  .frm file of CREATE TABLE t1 (a TIMESTAMP) PARTITION BY HASH(TO_DAYS(a));
mysql-test/t/partition.test:
  changed error to be more specific
mysql-test/t/partition_error.test:
  Added test (also for verifying behaviour of previously
  created tables which is no longer allowed).
  
  Updated expected errors in other places
sql/partition_info.cc:
  Added function report_part_expr_error to
  be able to return a more specific error.
  
  Renamed fix_func_partition to fix_partition_values
  since the function really fixes/checks the VALUES clause.
sql/partition_info.h:
  removed part_result_type, since it was unused.
  renamed fix_funk_partition->fix_partition_values
  added report_part_expr_error
sql/share/errmsg-utf8.txt:
  Added a more specific error.
sql/sql_partition.cc:
  made use of report_part_expr_error to get a more specific error.
sql/sql_yacc.yy:
  Changed error message to be more specific. And return an other error code.