We are trying to sort a lot of text/blob fields, 
so the buffer is indeed too small.
Memory available = thd->variables.sortbuff_size = 262144
min_sort_memory = param.sort_length*MERGEBUFF2 = 292245
So the decision to abort the query is correct.

filesort() calls my_error(), the error is reported.
But, since we have DELETE IGNORE ... the error is converted to a warning by
THD::raise_condition

filesort currently expects an error to be recorded in the THD diagnostics
area.
If we lift this restriction (remove the assert) we end up in the familiar
void Protocol::end_statement()
  default:
    DBUG_ASSERT(0);

The solution seems to be to call my_error(ME_FATALERROR) in filesort,
so that the error is propagated as an error rather than a warning.


mysql-test/r/filesort_debug.result:
  New test case.
mysql-test/t/filesort_debug.test:
  New test case.