Improve command help and don't allow management commands if the AS bot is not in the room

mautrix_telegram/commands.py

@@ -416,7 +416,8 @@ _**Telegram actions**: commands for using the bridge to interact with Telegram._
                       The type is either `group`, `supergroup` or `channel` (defaults to `group`).  
 **upgrade** - Upgrade a normal Telegram group to a supergroup.  
 **invitelink** - Get a Telegram invite link to the current chat.  
-**deleteportal** - Forget the current portal room.
+**deleteportal** - Forget the current portal room. Only works for group chats; to delete a private
+                   chat portal, simply leave the room.
 """
         return self.reply(management_status + help)
 

mautrix_telegram/matrix.py

@@ -153,7 +153,12 @@ class MatrixHandler:
         if message["msgtype"] != "m.text":
             return
 
-        is_management = len(self.az.intent.get_room_members(room)) == 2
+        try:
+            is_management = len(self.az.intent.get_room_members(room)) == 2
+        except MatrixRequestError:
+            # The AS bot is not in the room.
+            return
+
         if is_command or is_management:
             try:
                 command, arguments = text.split(" ", 1)