INSERT IGNORE had a pecular undocumented case that when one row was
inserted, there was an error rather than a warning.

As LOAD DATA IGNORE, UPDATE IGNORE, INSERT IGNORE SELECT, and INSERT
IGNORE VALUES (single row, for foreign key violation) all behave the same
way with a warning lets keep the behaviour normalized.

In compatibility, previously a error was generated, now a warning is
generated.

This behaviour is now consistent with MySQL-8.0 too.