Problem: An INSERT IGNORE statement trying to set a GEOMETRY NOT NULL
column to NULL would fail an assertion in debug mode.

INSERT IGNORE ignores a number of errors, instead converting the errors
to warnings, similar to non-strict mode. However, setting a non-nullable
GEOMETRY (or subtype) column to NULL is not allowed in non-strict mode
either, since there is no valid value to convert the NULL to. Still, the
error handler for INSERT IGNORE didn't flag an error, leading to
assertions testing thd->is_error() failing.

Fix: Create a new error code that is not ignored by the INSERT IGNORE
error handler.

Also raise errors in a few other cases where illegal NULL values where
silently converted to empty strings (and therefore invalid geometry
values).

Change-Id: I01593bf5a45355d7408129980db397877186ee02