Joining a room with no servers now returns a 502 rather than the actual error

[ara4n]

The logs have:

2020-02-03 22:40:25,470 - synapse.access.http.8095 - 233 - INFO - POST-3263108 - 92.234.32.44 - 8095 - Received request: POST /_matrix/client/r0/join/%23p2p%3Amatrix.org
2020-02-03 22:40:25,490 - synapse.http.client - 284 - INFO - POST-3263108 - Sending request POST http://10.103.0.1:9092/_synapse/replication/remote_join/%21NPmnTqbnUTWgPutRXK%3Amatrix.org/%40matthew%3Amatrix.org/RkfJCRkkee
2020-02-03 22:40:27,295 - synapse.http.client - 321 - INFO - POST-3263108 - Received response to POST http://10.103.0.1:9092/_synapse/replication/remote_join/%21NPmnTqbnUTWgPutRXK%3Amatrix.org/%40matthew%3Amatrix.org/RkfJCRkkee: 502
2020-02-03 22:40:27,296 - synapse.http.server - 81 - INFO - POST-3263108 - <XForwardedForRequest at 0x7fc1b1a64a20 method='POST' uri='/_matrix/client/r0/join/%23p2p%3Amatrix.org' clientproto='HTTP/1.1' site=8095> SynapseError: 502 - Failed to make_join via any server

But by the time it hits riot, it looks like this:

...which is less than helpful.

[t3chguy]

Looks fine here

[t3chguy]

Closing due to lack of repro