Draft: Fix dead DM when leaving from Gitter and trying to re-join

Fix dead DM when leaving from Gitter and trying to re-join.

Caused by:

• Accidentally using a lean troupe/room object so the feedback warning to the user about failing to join the room was never sent to the user
• We recently added support for syncing leave events to Matrix but this applied to all rooms we bridged so when you left a DM room, we also left on the Matrix side. But this meant that when you re-joined the room on the Gitter side, you could not get back in on the Matrix side.
• Also found that the logger.warn with the err didn't display it so now it's an object with exception which matches existing usage in the code base

Fix #2783 (closed)

Testing strategy

1. Start a DM conversation between a Matrix and Gitter user
2. Leave the DM on the Gitter side
3. From Gitter, open the DM again and send a message
4. Notice it failed the badger said Unable to invite Matrix user back to DM room. They probably won't know about the message you just sent. (caused by Failed to join room)