collateral changes:
* remove a test from innodb_virtual_basic that is already present in
gcol_keys_innodb
* set thd->abort_on_warning for inplace alter, just like it's set
for copy_data_between_tables - to have warnings converted into
errors identically in all alter algorithms
* don't ignore errors in TABLE::update_virtual_field
otherwise we'd need to store sql_mode *per vcol*
(consider CREATE INDEX...) and how SHOW CREATE TABLE would
support that?
Additionally, get rid of vcol::expr_str, just to make sure
the string is always generated and never leaked in the
original form.
