Display error message if no filenames have been specified.

SpritePacker/Program.cs

@@ -52,6 +52,11 @@ namespace SpritePacker
 			switch(programMode)
 			{
 				case ProgramMode.Normal:
+					if(values.Count == 0)
+					{
+						Console.Error.WriteLine("Error: No filenames specified!");
+						return 1;
+					}
 					RunNormal();
 					break;
 				case ProgramMode.DisplayHelpText: