Refactor exceptions

* Suffix names for custom exceptions with "Error"
* Introduce exceptions for when the coressponding encoder isn't found

spotdl/encode/tests/test_exceptions.py

@@ -0,0 +1,14 @@
+from spotdl.encode.exceptions import EncoderNotFoundError
+from spotdl.encode.exceptions import FFmpegNotFoundError
+from spotdl.encode.exceptions import AvconvNotFoundError
+
+import pytest
+
+
+class TestEncoderNotFoundSubclass:
+    def test_ffmpeg_not_found_subclass(self):
+        assert issubclass(FFmpegNotFoundError, EncoderNotFoundError)
+
+    def test_avconv_not_found_subclass(self):
+        assert issubclass(AvconvNotFoundError, EncoderNotFoundError)
+