Throw an exception when attempting to load an incompatible holder

Instead of a segfault. Fixes #751.

This covers the case of loading a custom holder from a default-holder
instance. Attempting to load one custom holder from a different custom
holder (i.e. not `std::unique_ptr`) yields undefined behavior, just as
#588 established for inheritance.

tests/test_smart_ptr.py

@@ -211,3 +211,12 @@ def test_move_only_holder():
     assert stats.alive() == 1
     del a
     assert stats.alive() == 0
+
+
+def test_smart_ptr_from_default():
+    from pybind11_tests.smart_ptr import HeldByDefaultHolder
+
+    instance = HeldByDefaultHolder()
+    with pytest.raises(RuntimeError) as excinfo:
+        HeldByDefaultHolder.load_shared_ptr(instance)
+    assert "Unable to load a custom holder type from a default-holder instance" in str(excinfo)