Updated error message to be more helpful

Also, added misssing test

_pytest/mark.py

@@ -169,7 +169,7 @@ class MarkGenerator:
 
     def __getattr__(self, name):
         if name[0] == "_":
-            raise AttributeError(name)
+            raise AttributeError("Marker name must NOT start with underscore")
         if hasattr(self, '_config'):
             self._check(name)
         return MarkDecorator(name)

testing/test_mark.py

@@ -15,6 +15,10 @@ class TestMark:
         mark = Mark()
         pytest.raises((AttributeError, TypeError), mark)
 
+    def test_pytest_mark_name_starts_with_underscore(self):
+        mark = Mark()
+        pytest.raises(AttributeError, getattr, mark, '_some_name')
+
     def test_pytest_mark_bare(self):
         mark = Mark()
         def f():