Refactor exceptions

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

spotdl/encode/tests/test_encode_base.py

@@ -0,0 +1,13 @@
+from spotdl.encode import EncoderBase
+
+import pytest
+
+def test_abstract_base_class_encoderbase():
+    encoder_path = "ffmpeg"
+    _loglevel = "-hide_banner -nostats -v panic"
+    _additional_arguments = ["-b:a", "192k", "-vn"]
+
+    with pytest.raises(TypeError):
+        # This abstract base class must be inherited from
+        # for instantiation
+        EncoderBase(encoder_path, _loglevel, _additional_arguments)