The optimizer must not continue executing the current query
if e.g. the storage engine reports an error.
This is somewhat hard to implement with Item::val_xxx()
because they do not have means to return error code.
This is why we need to check the thread's error state after
a call to one of the Item::val_xxx() methods.

Fixed store_key_item::copy_inner() to return an error state 
if an error happened during the call to Item::save_in_field() 
because it calls Item::val_xxx().
Also added similar checks to related places.