Fix refx/y validation and prepifg_tif_exists err msg.
Fix refx/y validation and prepifg_tif_exists err msg.

Reference pixel search trigger is now -1 -1. Conversion will only occur if refx/y != -1 and validation of search paramters will only occur if they refx/y == -1.

GeoscienceAustralia/PyRate,GeoscienceAustralia/PyRate