Display user friendly error if running against Synapse

[dagguh]

Describe the bug Threads are empty.

To Reproduce Steps to reproduce the behavior:

1. Go to a thread
2. See empty page

Expected behavior I'd expect to see the thread contents. Or to understand why I'm not able to see them. Or even what can I do to fix that.

Screenshots The page is empty. Also, the Firefox console logs an error:{ errcode: "M_UNRECOGNIZED", error: "Unrecognized request" }:

It's a body of a HTTP 400 response to a POST https://matrix.org/_matrix/client/unstable/event_relationships

• OS: iOS
• Browser Firefox 84.0.1

Additional context Logged in via federation to matrix.org.

[Half-Shot]

Cerulean does not support Synapse (which matrix.org runs) because it doesn't support some of the MSC implementations used (as seen in the wiki). You'll either have to use dendrite.matrix.org, or run your own Dendrite server with the MSCs enabled in the config.

@Kegsay do you want to keep this issue open because the application isn't showing an error, or should this be closed?

[dagguh]

Would be nice to see a message like:

Your homeserver matrix.org doesn't support XYZ.

[kegsay]

Yes, the bug here is that we should display this coherently to the user that it no worky.

[Half-Shot]

Relatedly we could get Cerulean to determine this dynamically if Dendrite implemented https://github.com/matrix-org/dendrite/issues/1696

[kegsay]

^ is the right approach here.