Handle all py::iterator errors

Before this, `py::iterator` didn't do any error handling, so code like:
```c++
for (auto item : py::int_(1)) {
    // ...
}
```
would just silently skip the loop. The above now throws `TypeError` as
expected. This is a breaking behavior change, but any code which relied
on the silent skip was probably broken anyway.

Also, errors returned by `PyIter_Next()` are now properly handled.

tests/test_sequences_and_iterators.py

@@ -11,7 +11,7 @@ def allclose(a_list, b_list, rel_tol=1e-05, abs_tol=0.0):
 
 
 def test_generalized_iterators():
-    from pybind11_tests import IntPairs
+    from pybind11_tests.sequences_and_iterators import IntPairs
 
     assert list(IntPairs([(1, 2), (3, 4), (0, 5)]).nonzero()) == [(1, 2), (3, 4)]
     assert list(IntPairs([(1, 2), (2, 0), (0, 3), (4, 5)]).nonzero()) == [(1, 2)]
@@ -23,7 +23,8 @@ def test_generalized_iterators():
 
 
 def test_sequence():
-    from pybind11_tests import Sequence, ConstructorStats
+    from pybind11_tests import ConstructorStats
+    from pybind11_tests.sequences_and_iterators import Sequence
 
     cstats = ConstructorStats.get(Sequence)
 
@@ -71,7 +72,7 @@ def test_sequence():
 
 
 def test_map_iterator():
-    from pybind11_tests import StringMap
+    from pybind11_tests.sequences_and_iterators import StringMap
 
     m = StringMap({'hi': 'bye', 'black': 'white'})
     assert m['hi'] == 'bye'
@@ -88,3 +89,27 @@ def test_map_iterator():
         assert m[k] == expected[k]
     for k, v in m.items():
         assert v == expected[k]
+
+
+def test_python_iterator_in_cpp():
+    import pybind11_tests.sequences_and_iterators as m
+
+    t = (1, 2, 3)
+    assert m.object_to_list(t) == [1, 2, 3]
+    assert m.object_to_list(iter(t)) == [1, 2, 3]
+    assert m.iterator_to_list(iter(t)) == [1, 2, 3]
+
+    with pytest.raises(TypeError) as excinfo:
+        m.object_to_list(1)
+    assert "object is not iterable" in str(excinfo.value)
+
+    with pytest.raises(TypeError) as excinfo:
+        m.iterator_to_list(1)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    def bad_next_call():
+        raise RuntimeError("py::iterator::advance() should propagate errors")
+
+    with pytest.raises(RuntimeError) as excinfo:
+        m.iterator_to_list(iter(bad_next_call, None))
+    assert str(excinfo.value) == "py::iterator::advance() should propagate errors"