slave.cc, sql_acl.cc:
- remove sql_print_error() for events that are not server errors


sql/slave.cc:
  Bug #20402: DROP USER failure logged as ERROR rather than WARNING
  remove sql_print_error() for events that are not server errors
sql/sql_acl.cc:
  Bug #20402: DROP USER failure logged as ERROR rather than WARNING
  remove sql_print_error() for events that are not server errors