Throw a more descriptive exception when pixelformat is unsupported.

src/bindings/csharp/openalpr-net/bitmapmat-net.h

@@ -65,7 +65,7 @@ namespace openalprnet {
 				channels = 4;
 				break;
 			default:
-				throw gcnew NotImplementedException();
+				throw gcnew NotSupportedException(bitmap->PixelFormat.ToString());
 			}
 
 			BitmapData^ bitmapData = bitmap->LockBits(