tests: make sure the type error is the one we want

tests/unit/commands/test_cmdutils.py

@@ -355,18 +355,27 @@ class TestRegister:
             """Blah."""
             pass
 
-        with pytest.raises(TypeError):
+        with pytest.raises(TypeError) as excinfo:
             fun = cmdutils.register()(fun)
 
+        expected = ("fun: handler has keyword only argument 'target' without "
+                    "default!")
+        assert str(excinfo.value) == expected
+
+
     def test_typed_keyword_only_without_default(self):
         # https://github.com/The-Compiler/qutebrowser/issues/1872
         def fun(*, target: int):
             """Blah."""
             pass
 
-        with pytest.raises(TypeError):
+        with pytest.raises(TypeError) as excinfo:
             fun = cmdutils.register()(fun)
 
+        expected = ("fun: handler has keyword only argument 'target' without "
+                    "default!")
+        assert str(excinfo.value) == expected
+
 
 class TestArgument: