Improve error handling if pipeline component is not callable (resolves #1911)

Also add help message if user accidentally calls nlp.add_pipe() with a string of a built-in component name.

spacy/language.py

@@ -236,6 +236,14 @@ class Language(object):
             >>> nlp.add_pipe(component, before='ner')
             >>> nlp.add_pipe(component, name='custom_name', last=True)
         """
+        if not hasattr(component, '__call__'):
+            msg = ("Not a valid pipeline component. Expected callable, but "
+                   "got {}. ".format(repr(component)))
+            if component in self.factories.keys():
+                msg += ("If you meant to add a built-in component, use "
+                        "create_pipe: nlp.add_pipe(nlp.create_pipe('{}'))"
+                        .format(component))
+            raise ValueError(msg)
         if name is None:
             if hasattr(component, 'name'):
                 name = component.name

spacy/tests/pipeline/test_pipe_methods.py

@@ -107,3 +107,9 @@ def test_add_lots_of_pipes(nlp, n_pipes):
     for i in range(n_pipes):
         nlp.add_pipe(lambda doc: doc, name='pipe_%d' % i)
     assert len(nlp.pipe_names) == n_pipes
+
+
+@pytest.mark.parametrize('component', ['ner', {'hello': 'world'}])
+def test_raise_for_invalid_components(nlp, component):
+    with pytest.raises(ValueError):
+        nlp.add_pipe(component)