configtypes: Handle {1} correctly.

qutebrowser/config/configtypes.py

@@ -874,7 +874,7 @@ class FormatString(BaseType):
         s = self.transform(value)
         try:
             return s.format(**{k: '' for k in self.fields})
-        except KeyError as e:
+        except (KeyError, IndexError) as e:
             raise configexc.ValidationError(value, "Invalid placeholder "
                                             "{}".format(e))
         except ValueError as e:
@@ -1089,7 +1089,7 @@ class SearchEngineUrl(BaseType):
             raise configexc.ValidationError(value, "must contain \"{}\"")
         try:
             value.format("")
-        except KeyError:
+        except (KeyError, IndexError) as e:
             raise configexc.ValidationError(
                 value, "may not contain {...} (use {{ and }} for literal {/})")
 

tests/config/test_configtypes.py

@@ -1501,6 +1501,7 @@ class TestSearchEngineUrl:
         'foo',  # no placeholder
         ':{}',  # invalid URL
         'foo{bar}baz{}',  # {bar} format string variable
+        '{1}{}',  # numbered format string variable
     ])
     def test_validate_invalid(self, klass, val):
         with pytest.raises(configexc.ValidationError):
@@ -1761,6 +1762,7 @@ class TestFormatString:
     @pytest.mark.parametrize('val', [
         '{foo} {bar} {baz}',
         '{foo} {bar',
+        '{1}',
         '',
     ])
     def test_validate_invalid(self, typ, val):