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/unit/test_music_server_client.gd

@@ -63,6 +63,39 @@ func test_invalid_json_does_not_call_back() -> void:
     assert_bool(called[0]).is_false()
 
 
+func test_failed_request_calls_on_failed() -> void:
+    _server.respond_bytes("/music/rand", PackedByteArray(), 503)
+    var outcome: Array[String] = []
+
+    MusicServerClient.fetch_random_song(false,
+            func(_body: PackedByteArray) -> void: outcome.append("done"),
+            func() -> void: outcome.append("failed"))
+
+    assert_array(outcome).contains_exactly(["failed"])
+
+
+func test_invalid_json_calls_on_failed() -> void:
+    _server.respond_bytes("/music/info", "not json".to_utf8_buffer())
+    var outcome: Array[String] = []
+
+    MusicServerClient.fetch_song_info(
+            func(_data: Variant) -> void: outcome.append("done"),
+            func() -> void: outcome.append("failed"))
+
+    assert_array(outcome).contains_exactly(["failed"])
+
+
+func test_successful_request_does_not_call_on_failed() -> void:
+    _server.respond_bytes("/music/addQue", PackedByteArray())
+    var outcome: Array[String] = []
+
+    MusicServerClient.add_to_queue(
+            func() -> void: outcome.append("done"),
+            func() -> void: outcome.append("failed"))
+
+    assert_array(outcome).contains_exactly(["done"])
+
+
 func test_send_calls_back_without_arguments() -> void:
     _server.respond_bytes("/music/reset", PackedByteArray())
     var called: Array[bool] = [false]