fix(reflectors): tolerate individual DNS resolution failures

Log and skip unresolved reflectors so startup can proceed with usable endpoints. Report an error when none remain for the selected protocol.

src/main.rs

@@ -174,7 +174,14 @@ fn resolve_reflectors(
         } else {
             0
         };
-        for endpoint in (definition.host.as_str(), port).to_socket_addrs()? {
+        let endpoints = match (definition.host.as_str(), port).to_socket_addrs() {
+            Ok(endpoints) => endpoints,
+            Err(error) => {
+                warn!("Failed to resolve reflector {}: {error}", definition.host);
+                continue;
+            }
+        };
+        for endpoint in endpoints {
             if protocol != ReflectorProtocol::Irtt && endpoint.is_ipv6() {
                 continue;
             }
@@ -199,6 +206,9 @@ fn setup_reflectors(settings: &Settings) -> anyhow::Result<ReflectorSetup> {
             .iter()
             .map(|(endpoint, definition)| (*endpoint, definition.clone())),
     );
+    if reflectors.is_empty() {
+        anyhow::bail!("no usable reflectors for measurement protocol {protocol}");
+    }
     let mut pool = configured.keys().copied().collect::<Vec<_>>();
     pool.sort();
     let mut initial = bootstrap.keys().copied().collect::<Vec<_>>();