Display an error message if an unknown flag is encountered.

SpritePacker/Program.cs

@@ -23,7 +23,7 @@ namespace SpritePacker
 		private static string commitHashFilename = "SpritePacker.latest-commit-hash.txt";
 		private static List<string> values = new List<string>();
 
-		public static void Main(string[] args)
+		public static int Main(string[] args)
 		{
 			string commitHash = Utilities.GetEmbeddedResourceContent(commitHashFilename).Trim();
 			Version += "-" + commitHash.Substring(commitHash.Length - 7);
@@ -39,6 +39,11 @@ namespace SpritePacker
 						programMode = ProgramMode.DisplayVersionText;
 						break;
 					default:
+						if(args[i].StartsWith("-"))
+						{
+							Console.Error.WriteLine("Error: Unrecognised flag '{0}'.", args[i]);
+							return 1;
+						}
 						values.Add(args[i]);
 						break;
 				}
@@ -66,6 +71,7 @@ namespace SpritePacker
 					break;
 			}
 
+			return 0;
 		}
 
 		public static void RunNormal()