Surface daemonized QEMU launch errors from the -D logfile

src/mcqemu/launcher.py

@@ -173,7 +173,13 @@ async def spawn_vm(cfg: VMConfig, config: Config) -> VMRecord:
     )
     _, stderr = await proc.communicate()
     if proc.returncode != 0:
-        detail = stderr.decode(errors="replace").strip() or f"exit code {proc.returncode}"
+        detail = stderr.decode(errors="replace").strip()
+        if not detail:
+            # Daemonized QEMU routes errors to the -D logfile, not stderr.
+            try:
+                detail = paths.qemu_log.read_text().strip().splitlines()[-1]
+            except (OSError, IndexError):
+                detail = f"exit code {proc.returncode}"
         raise ToolError(f"QEMU failed to launch VM {cfg.name!r}: {detail}")
 
     try: