Added py::register_exception for simple case (#296)

The custom exception handling added in PR #273 is robust, but is overly
complex for declaring the most common simple C++ -> Python exception
mapping that needs only to copy `what()`.  This add a simpler
`py::register_exception<CppExp>(module, "PyExp");` function that greatly
simplifies the common basic case of translation of a simple CppException
into a simple PythonException, while not removing the more advanced
capabilities of defining custom exception handlers.

tests/test_exceptions.py

@@ -22,7 +22,8 @@ def test_python_call_in_catch():
 
 
 def test_custom(msg):
-    from pybind11_tests import (MyException, throws1, throws2, throws3, throws4,
+    from pybind11_tests import (MyException, MyException5, MyException5_1,
+                                throws1, throws2, throws3, throws4, throws5, throws5_1,
                                 throws_logic_error)
 
     # Can we catch a MyException?"
@@ -49,3 +50,25 @@ def test_custom(msg):
     with pytest.raises(RuntimeError) as excinfo:
         throws_logic_error()
     assert msg(excinfo.value) == "this error should fall through to the standard handler"
+
+    # Can we handle a helper-declared exception?
+    with pytest.raises(MyException5) as excinfo:
+        throws5()
+    assert msg(excinfo.value) == "this is a helper-defined translated exception"
+
+    # Exception subclassing:
+    with pytest.raises(MyException5) as excinfo:
+        throws5_1()
+    assert msg(excinfo.value) == "MyException5 subclass"
+    assert isinstance(excinfo.value, MyException5_1)
+
+    with pytest.raises(MyException5_1) as excinfo:
+        throws5_1()
+    assert msg(excinfo.value) == "MyException5 subclass"
+
+    with pytest.raises(MyException5) as excinfo:
+        try:
+            throws5()
+        except MyException5_1 as e:
+            raise RuntimeError("Exception error: caught child from parent")
+    assert msg(excinfo.value) == "this is a helper-defined translated exception"