sporadically

There are two problems:

1. When closing temporary tables, during the THD clean up - and
   after the session connection was already closed, there is a
   chance we can push an error into the THD diagnostics area, if
   the writing of the implicit DROP event to the binary log fails
   for some reason. As a consequence an assertion can be
   triggered, because at that point the diagnostics area is
   already set.

2. Using push_warning with MYSQL_ERROR::WARN_LEVEL_ERROR is a 
   bug.

Given that close_temporary_tables is mostly called from
THD::cleanup - ie, with the session already closed, we fix
problem #1 by allowing the diagnostics area to be
overwritten. There is one other place in the code that calls
close_temporary_tables - while applying Start_log_event_v3. To
cover that case, we make close_temporary_tables to return the
error, thus, propagating upwards in the stack.

To fix problem #2, we replace push_warning with sql_print_error.