Refactor exceptions

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

spotdl/encode/exceptions.py

@@ -0,0 +1,20 @@
+class EncoderNotFoundError(Exception):
+    __module__ = Exception.__module__
+
+    def __init__(self, message=None):
+        super(EncoderNotFoundError, self).__init__(message)
+
+
+class FFmpegNotFoundError(EncoderNotFoundError):
+    __module__ = Exception.__module__
+
+    def __init__(self, message=None):
+        super(FFmpegNotFoundError, self).__init__(message)
+
+
+class AvconvNotFoundError(EncoderNotFoundError):
+    __module__ = Exception.__module__
+
+    def __init__(self, message=None):
+        super(AvconvNotFoundError, self).__init__(message)
+