portable: clarify error when detach fails to match to existing unit

Clarify that it's looking at attached units, not at units inside the referenced
image. Also take into account extensions.

NEWS

@@ -192,7 +192,7 @@ CHANGES WITH 251 in spe:
           hostnamed.
 
         * hostnamed's D-Bus interface gained a new method GetHardwareSerial()
-          for reading the hardware serial, as reportd by DMI.
+          for reading the hardware serial number, as reportd by DMI.
 
         * Two new hwdb files have been added. One lists "handhelds" (PDAs,
           calculators, etc.), the other AV production devices (DJ tables,
@@ -273,7 +273,7 @@ CHANGES WITH 251 in spe:
           the option is still understood to maximize compatibility.
 
         * 'bootctl list' gained support for a new --json= switch to output boot
-          menu entries in a simply JSON format.
+          menu entries in JSON format.
 
         * Services with Restart=always and a failing ExecCondition= will no
           longer be restarted, to bring ExecCondition= behaviour in line with
@@ -284,7 +284,7 @@ CHANGES WITH 251 in spe:
 
         * A new D-Bus property ControlGroupId is now exposed on service units,
           that encapsulates the service's numeric cgroup ID that newer kernels
-          maintain for each cgroup.
+          assign to each cgroup.
 
         * systemd-networkd gained a new [Bridge] Isolated=true|false setting
           that configures the eponymous kernel attribute on the bridge.

src/portable/portable.c

@@ -1520,6 +1520,7 @@ int portable_detach(
 
         _cleanup_(lookup_paths_free) LookupPaths paths = {};
         _cleanup_set_free_ Set *unit_files = NULL, *markers = NULL;
+        _cleanup_free_ char *extensions = NULL;
         _cleanup_closedir_ DIR *d = NULL;
         const char *where, *item;
         int ret = 0;
@@ -1674,8 +1675,17 @@ int portable_detach(
         return ret;
 
 not_found:
-        log_debug("No unit files associated with '%s' found. Image not attached?", name_or_path);
-        return sd_bus_error_setf(error, BUS_ERROR_NO_SUCH_UNIT, "No unit files associated with '%s' found. Image not attached?", name_or_path);
+        extensions = strv_join(extension_image_paths, ", ");
+        if (!extensions)
+                return -ENOMEM;
+
+        r = sd_bus_error_setf(error,
+                              BUS_ERROR_NO_SUCH_UNIT,
+                              "No unit files associated with '%s%s%s' found attached to the system. Image not attached?",
+                              name_or_path,
+                              isempty(extensions) ? "" : "' or any of its extensions '",
+                              isempty(extensions) ? "" : extensions);
+        return log_debug_errno(r, "%s", error->message);
 }
 
 static int portable_get_state_internal(