Say where the face models were looked for when they are not found

"The chosen detector is not installed" was the whole of what a user
saw, on a machine where the files were three directories away from
where the lookup went. The search order was in a doc comment and
nowhere a user could read it. Now a missing pair logs the detector
file it wanted and every directory it tried, which is what the first
Windows install needed and what the next misplaced download will.

ui/dr-ui/src/library.rs

@@ -4651,14 +4651,30 @@ pub fn face_models(
     account: &Account,
     detector: dr_types::FaceDetector,
 ) -> Option<(PathBuf, PathBuf)> {
-    let pair = |dir: PathBuf| {
+    let pair = |dir: &PathBuf| {
         let detector = dir.join(detector.file_name());
         let embedder = dir.join("arcface_mbf_b1.onnx");
         (detector.is_file() && embedder.is_file()).then_some((detector, embedder))
     };
-    pair(face_models_dir(account))
-        .or_else(|| pair(shared_face_models_dir()))
-        .or_else(|| system_face_models_dirs().into_iter().find_map(pair))
+    let mut searched = vec![face_models_dir(account), shared_face_models_dir()];
+    searched.extend(system_face_models_dirs());
+    let found = searched.iter().find_map(pair);
+    if found.is_none() {
+        // The settings page can only say "not installed". This is the line
+        // that says where it looked, which is the whole of what a user with
+        // the files in the wrong place needs — and the first thing to read
+        // when a freshly installed package reports no model.
+        log::warn!(
+            "face models: no directory holds both {} and arcface_mbf_b1.onnx; searched {}",
+            detector.file_name(),
+            searched
+                .iter()
+                .map(|d| d.display().to_string())
+                .collect::<Vec<_>>()
+                .join(", ")
+        );
+    }
+    found
 }
 
 /// The scene model, its vocabulary and its category descriptor, if all three