Retry failed song downloads instead of getting stuck

MusicServerClient functions now take an optional on_failed callback,
called when a request fails or returns invalid JSON. Before, a failed
request was silently dropped, so a failure while preloading a song left
Playlist.is_song_downloading set forever and no more songs were fetched.

Song fetching retries the step that failed (download, queue or info)
every song_fetch_retry_seconds, so the server and the game keep the same
playlist.

tests/integration/test_main_window.gd

@@ -78,3 +78,44 @@ func test_cannot_add_player_without_name() -> void:
     _add_player("")
 
     assert_array(Settings.player_array).is_empty()
+
+
+func test_failed_song_download_is_retried_until_the_server_answers() -> void:
+    _window.song_fetch_retry_seconds = 0.05
+    var songs_before: int = Playlist.song_object_array.size()
+    var mp3: PackedByteArray = FileAccess.get_file_as_bytes("res://tests/data/short.mp3")
+    _server.respond_bytes("/music/rand", PackedByteArray(), 503)
+    var rand_requests_before: int = _server.count("/music/rand")
+
+    _runner.invoke("fetch_song")
+    await await_millis(200)
+
+    assert_int(_server.count("/music/rand") - rand_requests_before).is_greater(1)
+    assert_int(Playlist.song_object_array.size()).is_equal(songs_before)
+    assert_bool(Playlist.is_song_downloading).is_true()
+
+    _server.respond_bytes("/music/rand", mp3)
+    await await_millis(200)
+
+    assert_int(Playlist.song_object_array.size()).is_equal(songs_before + 1)
+    assert_bool(Playlist.is_song_downloading).is_false()
+
+
+func test_failed_song_info_retries_only_that_step() -> void:
+    _window.song_fetch_retry_seconds = 0.05
+    var songs_before: int = Playlist.song_object_array.size()
+    _server.respond_bytes("/music/info", PackedByteArray(), 503)
+    var rand_before: int = _server.count("/music/rand")
+    var queue_before: int = _server.count("/music/addQue")
+
+    _runner.invoke("fetch_song")
+    await await_millis(200)
+    _next_song_is(SONG_INFO[1])
+    await await_millis(200)
+
+    # The song was downloaded and queued once; only the info request was repeated
+    assert_int(_server.count("/music/rand") - rand_before).is_equal(1)
+    assert_int(_server.count("/music/addQue") - queue_before).is_equal(1)
+    assert_int(Playlist.song_object_array.size()).is_equal(songs_before + 1)
+    assert_str(Playlist.song_object_array.back().song_title).is_equal("Dancing Mad")
+    assert_bool(Playlist.is_song_downloading).is_false()