configtypes: Handle invalid format syntax.

qutebrowser/config/configtypes.py

@@ -1092,6 +1092,8 @@ class SearchEngineUrl(BaseType):
         except (KeyError, IndexError) as e:
             raise configexc.ValidationError(
                 value, "may not contain {...} (use {{ and }} for literal {/})")
+        except ValueError as e:
+            raise configexc.ValidationError(value, str(e))
 
         url = QUrl(value.replace('{}', 'foobar'))
         if not url.isValid():

tests/config/test_configtypes.py

@@ -1502,6 +1502,7 @@ class TestSearchEngineUrl:
         ':{}',  # invalid URL
         'foo{bar}baz{}',  # {bar} format string variable
         '{1}{}',  # numbered format string variable
+        '{{}',  # invalid format syntax
     ])
     def test_validate_invalid(self, klass, val):
         with pytest.raises(configexc.ValidationError):