The problem was that mysql_upgrade failed because DROP DATABASE
refused to drop the 'performance_schema' database when the 
mysql.proc table definition was made temporarily invalid
by dump import.

This patch fixes the problem by adding the error resulting
from opening a damaged mysq.proc table (ER_CANNOT_LOAD_FROM_TABLE),
to the list of errors DROP DATABASE will ignore when trying
to lock stored procedures and functions before deletion.

This problem was a regression introduced by the patch for
Bug#57663.

Test case added to sp-destruct.test.