ENH: Clarify usage, skip intensity image if it is "none" or "na"

Skips intensity image if placeholder "none" or "na" is used
(case insensitive). Exit with error if some other value is given
and the file cannot be read.

Examples/LabelGeometryMeasures.cxx

@@ -26,6 +26,7 @@
 #include <iostream>
 #include <ostream>
 #include <sstream>
+#include <string>
 
 namespace ants
 {
@@ -46,13 +47,17 @@ LabelGeometryMeasures(int argc, char * argv[])
   bool                            intensityImageUsed = false;
   if (argc > 3)
   {
-    try
+    std::string intensityImageFile = argv[3];
+    ConvertToLowerCase(intensityImageFile);
+    if ( intensityImageFile.compare("na") != 0 && intensityImageFile.compare("none") != 0 )
     {
-      ReadImage<RealImageType>(intensityImage, argv[3]);
+      if (!ReadImage<RealImageType>(intensityImage, intensityImageFile.c_str()))
+      {
+        std::cout << "Input intensity image " << intensityImageFile << " could not be read." << std::endl;
+        return EXIT_FAILURE;
+      }
       intensityImageUsed = true;
     }
-    catch (...)
-    {}
   }
 
   bool outputCSVFormat = false;
@@ -350,10 +355,8 @@ LabelGeometryMeasures(std::vector<std::string> args, std::ostream * itkNotUsed(o
 
   if (argc < 3)
   {
-    std::cout << "Usage 1: " << argv[0] << " imageDimension labelImage [intensityImage=none] [csvFile]" << std::endl;
-    //     std::cout << "Usage 2: " << argv[0] << " X singleLabelImage outputTransform <doReflection=1> <scaleFactor=1>"
-    //     << std::endl;
-
+    std::cout << "Usage 1: " << argv[0] << " imageDimension labelImage [intensityImage] [csvFile]" << std::endl;
+    std::cout << "To output to csvFile without an intensity image, use \"none\" or \"na\" as the third arg" << std::endl;
     if (argc >= 2 && (std::string(argv[1]) == std::string("--help") || std::string(argv[1]) == std::string("-h")))
     {
       return EXIT_SUCCESS;