You could use try/catch statements where you use the xlsread function. Edit the code in your callback function like this:
try
NumData = xlsread(FileName); %wrap your xlsread with try/catch statement
catch
set(handles.text1, 'String', 'Error loading data!'); %update your status text
drawnow; %redraw GUI to ensure the new string shows
return %end the callback function prematurely due to error
end
