"we couldn't reach this homeserver" is incredibly vague and hard to debug

### Steps to reproduce

1. i'm trying to login to matrix.debian.social with element x
2. i follow the login process and enter the homeserver URL and fail

### Outcome

#### What did you expect?

I expected to be able to login to the homeserver. It's running synapse 1.127.1 so it should have sliding sync (right?).

#### What happened instead?

Instead, it now fails on the MAS stuff, as it gets a 404 on `/_matrix/client/unstable/org.matrix.msc2965/auth_metadata`.

It seems to me this could be relayed back to the user. There are a couple of similar bug reports in this issue tracker, yet they all vary subtly:

- #1097 is about missing sliding sync support (same error message!)
- #3447 seems to be about proxy issues
- #954 is similarly unclear but seemed to be about SSO

In generaly, percolate back to the user the actual error, including the HTTP status code and the URL it tried to reach. Telling users to "contact their homeserver admin" is not really useful, especially when the user *is* the homeserver admin. I only found out what the issue was by connecting to a separate IP (so i could find the traffic in the log) and grepping `homeserver.log`, which is not really intuitive.

I am also, fundamentally, puzzled that Element X already *requires* MAS given that this was deployed only *last week* on Matrix.org. Maybe I'm missing something?

### Your phone model

irrelevant

### Operating system version

Android 15 / CalyxOS

### Application version and app store

element x from f-droid

### Homeserver

1.127.1

### Will you send logs?

Yes

### Are you willing to provide a PR?

No