Fix skip decorator.

If called w/o argument would just skip the function always.
Now raises if no message is passed.

IPython/testing/decorators.py

@@ -245,8 +245,10 b' def skip(msg=None):'
          Decorator, which, when applied to a function, causes SkipTest
          to be raised, with the optional message added.
       """
-
-    return skipif(True,msg)
+    if msg and not isinstance(msg, str):
+        raise ValueError('invalid object passed to `@skip` decorator, did you '
+                         'meant `@skip()` with brackets ?')
+    return skipif(True, msg)
 
 
 def onlyif(condition, msg):

IPython/testing/tests/test_decorators.py

@@ -46,7 +46,7 b' def trivial():'
     pass
 
 
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 def test_deliberately_broken():
     """A deliberately broken test - we want to skip this one."""
     1/0