Problem: mysql_upgrade treats a warning with severity 'ERROR' as a real query execution error, so it stops when such a warning appears. In this particular bug, musql_upgrade got a warning after "DROP DATABASE IF EXISTS performance_schema" query and finished, although the database is dropped, and the next mysql_upgrade run will go with no problem.

Fix: relax it, don't treat warnings as query execution errors.

Note: all warning messages are prepending with 'non fatal' from now on.