A delivery processor may want to finish the delivery early. Change the
return value from VDP_bytes and the delivery processors into 3
categories. 0 means continue, negative is error and positive is
finished.

The latching of errors (to help ESI delivery continue while still
recording the error) will latch the lowest non-zero value observed.

VDP_gunzip is fixed to not unconditionally return errors.

Also add some text to explain the behaviour of VDP_bytes and its
delivery processors.