An internal routine (in this case finDiffEvalAndChkErr) is calling your routine with only one output. For a regular function that returns two outputs, this is fine, as you are allowed to do this (the second output gets ignored).
But for your function with deal, you are requiring it to always be called with two outputs, rather than optionally.
I suggest you just resign yourself to writing a wrapper function for your two routines (you can also save yourself evaluating the gradient too if it's not required).
function [f, g] = fun_all(x)
f = fun_obj(x);
if nargout == 2
g = fun_grad(x);
end
end
