Inviting users by ID who aren't on the system silently fails

[matrixbot]

Created by @ matthew:matrix.org.

If I invite @89y245tr8egfs:matrix.org to a room, surely it should tell me that that user doesn't exist

[ara4n]

@erikjohnston should I be surprised that this doesn't work on synapse?

[erikjohnston]

Clients should be able to do a GET /_matrix/client/r0/profile/<user_id> to check for the existence of users, it will return a 404 if user doesn't exist.

[lukebarnard1]

This looks actionable. Let's poke matrixClient.getProfileInfo when we display tiles in the invite dialog and indicate whether the user could be found. If not, we don't allow the invite to succeed.