Problem:

Suppose there are two tables in the database related through a foreign key
constraint - the parent table and the child table.  Suppose there is an ALTER
TABLE on the child table which gets interrupted in such a way that the child
table is not available any more.

After crash recovery, an ALTER TABLE on the parent table identifies that its
foreign keys cannot be loaded.  This results in an error and a debug assert.

Solution:

Remove the debug assert and change error to a warning.

rb#8658 approved by Marko.