Add the root_dir value to the logging message in case of non complian…

…t preferred_dir (#804)

jupyter_server/serverapp.py

@@ -1605,7 +1605,10 @@ def _preferred_dir_validate(self, proposal):
         # preferred_dir must be equal or a subdir of root_dir
         if not value.startswith(self.root_dir):
             raise TraitError(
-                trans.gettext("preferred_dir must be equal or a subdir of root_dir: '%r'") % value
+                trans.gettext(
+                    "preferred_dir must be equal or a subdir of root_dir. preferred_dir: '%r' root_dir: '%r'"
+                )
+                % (value, self.root_dir)
             )
 
         return value

tests/test_serverapp.py

@@ -309,7 +309,7 @@ def test_invalid_preferred_dir_not_root_subdir(tmp_path, jp_configurable_servera
     with pytest.raises(TraitError) as error:
         app = jp_configurable_serverapp(root_dir=path, preferred_dir=not_subdir_path)
 
-    assert "preferred_dir must be equal or a subdir of root_dir:" in str(error)
+    assert "preferred_dir must be equal or a subdir of root_dir. " in str(error)
 
 
 def test_invalid_preferred_dir_not_root_subdir_set(tmp_path, jp_configurable_serverapp):
@@ -321,7 +321,7 @@ def test_invalid_preferred_dir_not_root_subdir_set(tmp_path, jp_configurable_ser
     with pytest.raises(TraitError) as error:
         app.preferred_dir = not_subdir_path
 
-    assert "preferred_dir must be equal or a subdir of root_dir:" in str(error)
+    assert "preferred_dir must be equal or a subdir of root_dir. " in str(error)
 
 
 def test_observed_root_dir_updates_preferred_dir(tmp_path, jp_configurable_serverapp):