improve error msg on timeout opening stream

Before, it'd look like the following, as mentioned in
#331:

```
2024-12-09T20:53:24.853572 WARN s-CP3PRO-main streamer{stream="CP3PRO-main"}: moonfire_nvr::streamer: sleeping for 1 s after error err=UNKNOWN
caused by: deadline has elapsed
```

...which isn't as clear as it could be, particularly if you don't notice
the `caused by` bit. After, it should look more like the following:

```
2024-12-09T20:53:24.853572 WARN s-CP3PRO-main streamer{stream="CP3PRO-main"}: moonfire_nvr::streamer: sleeping for 1 s after error err=DEADLINE_EXCEEDED unable to play stream and get first frame within PT30S
caused by: deadline has elapsed
```

CHANGELOG.md

@@ -11,6 +11,7 @@ even on minor releases, e.g. `v0.7.5` -> `v0.7.6`.
 ## unreleased
 
 *   bump minimum Rust version to 1.81.
+*   improve error message on timeout opening stream.
 
 ## v0.7.17 (2024-09-03)
 

server/src/stream.rs

@@ -111,7 +111,13 @@ impl Opener for RealOpener {
                 ),
             )
             .expect("RetinaStream::play task panicked, see earlier error")
-            .map_err(|e| err!(Unknown, source(e)))??;
+            .map_err(|e| {
+                err!(
+                    DeadlineExceeded,
+                    msg("unable to play stream and get first frame within {RETINA_TIMEOUT:?}"),
+                    source(e),
+                )
+            })??;
         Ok(Box::new(RetinaStream {
             inner: Some(inner),
             rt_handle,
@@ -302,7 +308,7 @@ impl Stream for RetinaStream {
                     .map_err(|e| {
                         err!(
                             DeadlineExceeded,
-                            msg("timeout getting next frame"),
+                            msg("unable to get next frame within {RETINA_TIMEOUT:?}"),
                             source(e)
                         )
                     })??;