Improve errors when MAS contacts the Synapse homeserver (#2794)

* Add some drive-by docstrings

* Change text rendering of catch_http_codes::HttpError

Using `#[source]` is unnatural here because it makes it look like
two distinct errors (one being a cause of the other),
when in reality it is just one error, with 2 parts.

Using `Display` formatting for that leads to a more natural error.

* Add constraints to `catch_http_code{,s}` methods

Not strictly required, but does two things:

- documents what kind of function is expected
- provides a small extra amount of type enforcement at the call site,
  rather than later on when you find the result doesn't implement Service

* Add a `catch_http_errors` shorthand

Nothing major, just a quality of life improvement so you don't have to
repetitively write out what a HTTP error is

* Unexpected error page: remove leading whitespace from preformatted 'details' section

The extra whitespace was probably unintentional and makes the error harder to read,
particularly when it wraps onto a new line unnecessarily

* Capture and log Matrix errors received from Synapse

* Drive-by clippy fix: use clamp instead of min().max()

* Convert `err(Display)` to `err(Debug)` for `anyhow::Error`s in matrix-synapse support module

crates/axum-utils/src/language_detection.rs

@@ -109,7 +109,7 @@ impl Header for AcceptLanguage {
                     let quality = std::str::from_utf8(quality).map_err(|_e| Error::invalid())?;
                     let quality = quality.parse::<f64>().map_err(|_e| Error::invalid())?;
                     // Bound the quality between 0 and 1
-                    let quality = quality.min(1_f64).max(0_f64);
+                    let quality = quality.clamp(0_f64, 1_f64);
 
                     // Make sure the iterator is empty
                     if it.next().is_some() {