rfc5321: improve error message for bad IDNA domains

Previously, we'd use the default presentation for the Error type
reported by the idna crate.  Unfortunately, that type holds no
context: it is a zero-sized empty struct type, and it chooses
to render itself as `Errors`, which isn't super helpful, but
it is hard to convey every possible error clause from the underlying
spec, so I understand why this is this way.

This commit handles that error case to present a slightly more
informative error message.

It doesn't provide context on what specifically is bad about
the input, but it at least helps to characterize that the domain
is bad, rather than imply that we don't know how to deal with
punycode at all.

The specific problem with the domain in the included test
case is most likely BIDI related per discussion in
servo/rust-url#489

Author: wez

crates/rfc5321/src/parser.rs

@@ -355,7 +355,7 @@ impl Domain {
     pub fn name(name: &str) -> Result<Self, String> {
         match idna::domain_to_ascii(name) {
             Ok(name) => Ok(Self::Name(name)),
-            Err(err) => Err(format!("{err:#}")),
+            Err(_empty_error_type) => Err(format!("invalid IDNA domain {name}")),
         }
     }
 }
@@ -588,6 +588,16 @@ mod test {
         );
     }
 
+    #[test]
+    fn parse_rcpt_to_punycode() {
+        assert_eq!(
+            Parser::parse_command("Rcpt To:<[email protected]>")
+                .unwrap_err()
+                .to_string(),
+            "invalid IDNA domain 4bed.xn--5dbhlacyps5bf4a.com"
+        );
+    }
+
     #[test]
     fn parse_rcpt_to() {
         assert_eq!(