Add tests for encoders

and some refactoring

spotdl/encode/encoders/tests/test_ffmpeg.py

@@ -1,5 +1,5 @@
 from spotdl.encode import EncoderBase
-# from spotdl.encode import exceptions
+from spotdl.encode.exceptions import FFmpegNotFoundError
 from spotdl.encode.encoders import EncoderFFmpeg
 
 import pytest
@@ -9,6 +9,10 @@ class TestEncoderFFmpeg:
     def test_subclass(self):
         assert issubclass(EncoderFFmpeg, EncoderBase)
 
+    def test_ffmpeg_not_found_error(self):
+        with pytest.raises(FFmpegNotFoundError):
+            EncoderFFmpeg(encoder_path="/a/nonexistent/path")
+
 
 class TestEncodingDefaults:
     def m4a_to_mp3_encoder(input_file, output_file):