📝 Add docstrings

PlexBot/exceptions.py

@@ -1,6 +1,10 @@
 class MediaNotFoundError(Exception):
+    """Raised when a PlexAPI media resource cannot be found."""
+
     pass
 
 
 class VoiceChannelError(Exception):
+    """Raised when user is not connected to a voice channel."""
+
     pass