Handle room conflicts gracefully

[kittykat]

We don't want the bot trying to create rooms that already exist (e.g. https://matrix.to/#/#INFODESK:fosdem.org).

The bot should check if the target alias already exists before creating the room and if it does, then skip room creation but share this info with the user

[squahtx]

#77 will allow us to reuse the #infodesk room instead of creating a new one.