This makes the zend_error_cb API simpler, and avoid formatting
the same message in multiple places.
It should be noted that the passed zend_string is always
non-persistent, so if you want to store it persistently somewhere,
you may still need to duplicate it.
The last_error_message is cleared a bit more aggressive, to make
sure it doesn't hang around across allocator life-cycles.
Closes GH-5639.