Update frame.cpp

Image seemed to be ok but still exception was being thrown.

svo/src/frame.cpp

@@ -48,7 +48,7 @@ void Frame::initFrame(const cv::Mat& img)
 {
   // check image
   if(img.empty() || img.type() != CV_8UC1 || img.cols != cam_->width() || img.rows != cam_->height())
-    throw std::runtime_error("Frame: provided image has not the same size as the camera model");
+    throw std::runtime_error("Frame: provided image has not the same size as the camera model or image is not grayscale");
 
   // Set keypoints to NULL
   std::for_each(key_pts_.begin(), key_pts_.end(), [&](Feature* ftr){ ftr=NULL; });