not only print file-not-found as a warning but also print the name of the problematic file and the uri used for the open attempt

modules/videoio/src/cap_ffmpeg_impl.hpp

@@ -586,6 +586,7 @@ bool CvCapture_FFMPEG::open( const char* _filename )
     if (err < 0)
     {
         CV_WARN("Error opening file");
+        CV_WARN(_filename);
         goto exit_func;
     }
     err =

modules/videoio/src/cap_gstreamer.cpp

@@ -601,6 +601,8 @@ bool CvCapture_GStreamer::open( int type, const char* filename )
             else
             {
                 CV_WARN("GStreamer: Error opening file\n");
+                CV_WARN(filename);
+                CV_WARN(uri);
                 close();
                 return false;
             }