Currently when we need to use a remote grid that can't be opened, we
return HUGE_VAL coordinates values and a proj_errno = PROJ_ERR_OTHER_NETWORK_ERROR,
(I guess) as expected... But if we do following proj_trans() calls on the same
transformation object, the grid transformation is ignored and we
fallback to other methods (Helmert, ballpark, etc.).
Fix that by consistently returning the same error values as the initial
failed call.

Fixes pyproj4/pyproj#705

…iled to open from the same PJ* instance