don't swallow Python errors in tree iteration

Summary:
During the sequence of events that led up to S169085 we tried an hg
push that resulted in an error that looked like this: "RuntimeError:
std::exception". I believe that error was caused by an uncaught
pyexception, replacing Python's current thread exception with a
useless one.

Catch the pyexception and deal with it appropriately.

Reviewed By: quark-zju

Differential Revision: D13112735

fbshipit-source-id: 301d899543ae95084b890c19b00322e69ded07b2

hgext/extlib/cstore/py-treemanifest.h

@@ -298,6 +298,8 @@ static PyObject* newtreeiter_iternext(py_newtreeiter* self) {
           p2Node,
           (Py_ssize_t)BIN_NODE_SIZE);
     }
+  } catch (const pyexception&) {
+    return NULL;
   } catch (const std::exception& ex) {
     PyErr_SetString(PyExc_RuntimeError, ex.what());
     return NULL;