refactoring catching all exceptions as const ref

samples/cpp/pca.cpp

@@ -141,7 +141,7 @@ int main(int argc, char** argv)
     // Read in the data. This can fail if not valid
     try {
         read_imgList(imgList, images);
-    } catch (cv::Exception& e) {
+    } catch (const cv::Exception& e) {
         cerr << "Error opening file \"" << imgList << "\". Reason: " << e.msg << endl;
         exit(1);
     }