ao_alsa: make partial writes an error message

And I think "partial write" is easier to understand than "short write".

audio/out/ao_alsa.c

@@ -1086,7 +1086,7 @@ static bool audio_write(struct ao *ao, void **data, int samples)
 
     CHECK_ALSA_ERROR("pcm write error");
     if (err >= 0 && err != samples) {
-        MP_WARN(ao, "unexpected short write (%d of %d frames), dropping audio\n",
+        MP_ERR(ao, "unexpected partial write (%d of %d frames), dropping audio\n",
                (int)err, samples);
     }