add debug logs on conn error/eof

The caller gets this information and may do its own logging with more
context. But...sometimes the caller doesn't, and being able to turn
this on is handy then, so log at low priority. (debug; I considered
trace even.)

src/client/mod.rs

@@ -2358,8 +2358,14 @@ impl futures::Stream for Session<Playing> {
                             request);
                     }
                 },
-                Poll::Ready(Some(Err(e))) => return Poll::Ready(Some(Err(e))),
-                Poll::Ready(None) => return Poll::Ready(None),
+                Poll::Ready(Some(Err(e))) => {
+                    debug!("RTSP connection error: {:?}", e);
+                    return Poll::Ready(Some(Err(e)))
+                },
+                Poll::Ready(None) => {
+                    debug!("Server closed RTSP connection");
+                    return Poll::Ready(None)
+                },
                 std::task::Poll::Pending => {}
             }