Fix exception messages in Command

The messages weren't updated in ac78039171.

qutebrowser/commands/command.py

@@ -157,18 +157,18 @@ class Command:
         if 'self' in signature.parameters:
             if self._instance is None:
                 raise TypeError("{} is a class method, but instance was not "
-                                "given!".format(self.name[0]))
+                                "given!".format(self.name))
             arg_info = self.get_arg_info(signature.parameters['self'])
             if arg_info.value is not None:
                 raise TypeError("{}: Can't fill 'self' with value!"
                                 .format(self.name))
         elif 'self' not in signature.parameters and self._instance is not None:
             raise TypeError("{} is not a class method, but instance was "
-                            "given!".format(self.name[0]))
+                            "given!".format(self.name))
         elif any(param.kind == inspect.Parameter.VAR_KEYWORD
                  for param in signature.parameters.values()):
             raise TypeError("{}: functions with varkw arguments are not "
-                            "supported!".format(self.name[0]))
+                            "supported!".format(self.name))
 
     def get_arg_info(self, param):
         """Get an ArgInfo tuple for the given inspect.Parameter."""

tests/unit/api/test_cmdutils.py

@@ -190,6 +190,27 @@ class TestRegister:
         args, kwargs = cmd._get_call_args(win_id=0)
         fun(*args, **kwargs)
 
+    def test_self_without_instance(self):
+        with pytest.raises(TypeError, match="fun is a class method, but "
+                           "instance was not given!"):
+            @cmdutils.register()
+            def fun(self):
+                """Blah."""
+
+    def test_instance_without_self(self):
+        with pytest.raises(TypeError, match="fun is not a class method, but "
+                           "instance was given!"):
+            @cmdutils.register(instance='inst')
+            def fun():
+                """Blah."""
+
+    def test_var_kw(self):
+        with pytest.raises(TypeError, match="fun: functions with varkw "
+                           "arguments are not supported!"):
+            @cmdutils.register()
+            def fun(**kwargs):
+                """Blah."""
+
     def test_partial_arg(self):
         """Test with only some arguments decorated with @cmdutils.argument."""
         @cmdutils.register()