No functionality added or changed.
This is a pre-requisite for the fix for Bug#12713 Error in a stored 
function called from a SELECT doesn't cause ROLLBACK of statem

Address post-review comments.


include/sql_common.h:
  Declare auxiliary functions to manipulate mysql.net.last_er* and 
  mysql_stmt.last_er*
libmysql/libmysql.c:
  Use helper functions to set an error in MYSQL or MYSQL_STMT
libmysqld/lib_sql.cc:
  Use helper functions to set an error in MYSQL or MYSQL_STMT
sql-common/client.c:
  Use helper functions to set an error in MYSQL or MYSQL_STMT