Catch AttributeError for FormatString configtype

(cherry picked from commit b171f3c0f2)

qutebrowser/config/configtypes.py

@@ -1567,7 +1567,7 @@ class FormatString(BaseType):
 
         try:
             value.format(**{k: '' for k in self.fields})
-        except (KeyError, IndexError) as e:
+        except (KeyError, IndexError, AttributeError) as e:
             raise configexc.ValidationError(value, "Invalid placeholder "
                                             "{}".format(e))
         except ValueError as e:

tests/unit/config/test_configtypes.py

@@ -1832,6 +1832,8 @@ class TestFormatString:
         '{foo} {bar} {baz}',
         '{foo} {bar',
         '{1}',
+        '{foo.attr}',
+        '{foo[999]}',
     ])
     def test_to_py_invalid(self, typ, val):
         with pytest.raises(configexc.ValidationError):