Merge pull request #395 from aldanor/feature/error-already-set-message

error_already_set improvements
diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h
index 080b2e0..438f7cf 100644
--- a/include/pybind11/cast.h
+++ b/include/pybind11/cast.h
@@ -160,7 +160,7 @@
     PYBIND11_NOINLINE bool load(handle src, bool convert) {
         if (!src || !typeinfo)
             return false;
-        if (src.ptr() == Py_None) {
+        if (src.is_none()) {
             value = nullptr;
             return true;
         } else if (PyType_IsSubtype(Py_TYPE(src.ptr()), typeinfo->type)) {
@@ -185,7 +185,7 @@
                                          const void *existing_holder = nullptr) {
         void *src = const_cast<void *>(_src);
         if (src == nullptr)
-            return handle(Py_None).inc_ref();
+            return none();
 
         auto &internals = get_internals();
 
@@ -413,7 +413,7 @@
 public:
     bool load(handle, bool) { return false; }
     static handle cast(void_type, return_value_policy /* policy */, handle /* parent */) {
-        return handle(Py_None).inc_ref();
+        return none();
     }
     PYBIND11_TYPE_CASTER(void_type, _("None"));
 };
@@ -425,7 +425,7 @@
     bool load(handle h, bool) {
         if (!h) {
             return false;
-        } else if (h.ptr() == Py_None) {
+        } else if (h.is_none()) {
             value = nullptr;
             return true;
         }
@@ -451,7 +451,7 @@
         if (ptr)
             return capsule(ptr).release();
         else
-            return handle(Py_None).inc_ref();
+            return none();
     }
 
     template <typename T> using cast_op_type = void*&;
@@ -563,12 +563,12 @@
 template <> class type_caster<char> : public type_caster<std::string> {
 public:
     bool load(handle src, bool convert) {
-        if (src.ptr() == Py_None) return true;
+        if (src.is_none()) return true;
         return type_caster<std::string>::load(src, convert);
     }
 
     static handle cast(const char *src, return_value_policy /* policy */, handle /* parent */) {
-        if (src == nullptr) return handle(Py_None).inc_ref();
+        if (src == nullptr) return none();
         return PyUnicode_FromString(src);
     }
 
@@ -586,12 +586,12 @@
 template <> class type_caster<wchar_t> : public type_caster<std::wstring> {
 public:
     bool load(handle src, bool convert) {
-        if (src.ptr() == Py_None) return true;
+        if (src.is_none()) return true;
         return type_caster<std::wstring>::load(src, convert);
     }
 
     static handle cast(const wchar_t *src, return_value_policy /* policy */, handle /* parent */) {
-        if (src == nullptr) return handle(Py_None).inc_ref();
+        if (src == nullptr) return none();
         return PyUnicode_FromWideChar(src, (ssize_t) wcslen(src));
     }
 
@@ -762,7 +762,7 @@
     bool load(handle src, bool convert) {
         if (!src || !typeinfo) {
             return false;
-        } else if (src.ptr() == Py_None) {
+        } else if (src.is_none()) {
             value = nullptr;
             return true;
         } else if (PyType_IsSubtype(Py_TYPE(src.ptr()), typeinfo->type)) {
diff --git a/include/pybind11/functional.h b/include/pybind11/functional.h
index 0df8129..7f032fc 100644
--- a/include/pybind11/functional.h
+++ b/include/pybind11/functional.h
@@ -20,7 +20,7 @@
     typedef typename std::conditional<std::is_same<Return, void>::value, void_type, Return>::type retval_type;
 public:
     bool load(handle src_, bool) {
-        if (src_.ptr() == Py_None)
+        if (src_.is_none())
             return true;
 
         src_ = detail::get_function(src_);
@@ -62,7 +62,7 @@
     template <typename Func>
     static handle cast(Func &&f_, return_value_policy policy, handle /* parent */) {
         if (!f_)
-            return handle(Py_None).inc_ref();
+            return none();
 
         auto result = f_.template target<Return (*)(Args...)>();
         if (result)
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 0d16c7a..dd04e95 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1143,7 +1143,7 @@
     if (!nurse || !patient)
         pybind11_fail("Could not activate keep_alive!");
 
-    if (patient.ptr() == Py_None || nurse.ptr() == Py_None)
+    if (patient.is_none() || nurse.is_none())
         return; /* Nothing to keep alive or nothing to be kept alive by */
 
     cpp_function disable_lifesupport(
diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h
index 51f1629..ba5d2c4 100644
--- a/include/pybind11/pytypes.h
+++ b/include/pybind11/pytypes.h
@@ -40,6 +40,7 @@
     inline detail::accessor attr(const char *key) const;
     inline pybind11::str str() const;
     inline pybind11::str repr() const;
+    bool is_none() const { return m_ptr == Py_None; }
     template <typename T> T cast() const;
     template <return_value_policy policy = return_value_policy::automatic_reference, typename ... Args>
     #if __cplusplus > 201103L