Show streamlink/ffmpeg stderr for debugging, handle startup errors

- stderr from both processes now displayed in console
- streamlink checked for early exit before starting ffmpeg
- RuntimeError shown to user if stream unavailable

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.py

@@ -28,20 +28,26 @@ async def run(config) -> None:
 
     frame_number = 0
 
-    async for frame_data in capture_frames(
-        config.channel, config.quality, config.interval
-    ):
-        frame_number += 1
-        console.print(f"[dim]Captured frame #{frame_number}, analyzing...[/dim]")
+    try:
+        async for frame_data in capture_frames(
+            config.channel, config.quality, config.interval
+        ):
+            frame_number += 1
+            console.print(f"[dim]Captured frame #{frame_number}, analyzing...[/dim]")
 
-        try:
-            description = await analyzer.analyze_frame(frame_data)
-        except Exception as e:
-            console.print(f"[bold red]Analysis error:[/bold red] {e}")
-            continue
+            try:
+                description = await analyzer.analyze_frame(frame_data)
+            except Exception as e:
+                console.print(f"[bold red]Analysis error:[/bold red] {e}")
+                continue
 
-        print_description(description, frame_number)
-        await log_description(config.log_file, description, frame_number)
+            print_description(description, frame_number)
+            await log_description(config.log_file, description, frame_number)
+    except RuntimeError as e:
+        console.print(f"[bold red]Error:[/bold red] {e}")
+    finally:
+        if frame_number == 0:
+            console.print("[bold yellow]No frames were captured.[/bold yellow]")
 
 
 def main() -> None: