Improving errors when an alias is not found

src/build_system.ml

@@ -118,11 +118,18 @@ module Build_error = struct
     raise (E { backtrace; dep_path; exn })
 end
 
+let describe_target fn =
+  match Path.extract_build_context fn with
+  | Some (".aliases", dir) ->
+    sprintf "alias %s" (Path.to_string dir)
+  | _ ->
+    Path.to_string fn
+
 let wait_for_file t fn ~targeting =
   match Hashtbl.find t.files fn with
   | None ->
     if Path.is_in_build_dir fn then
-      die "no rule found for %s" (Path.to_string fn)
+      die "no rule found for %s" (describe_target fn)
     else if Path.exists fn then
       return ()
     else