Add getters for exception type, value and traceback

Author: martinRenou

include/pybind11/pytypes.h

@@ -346,6 +346,10 @@ class error_already_set : public std::runtime_error {
     /// the given tuple.
     bool matches(handle ex) const { return PyErr_GivenExceptionMatches(ex.ptr(), type.ptr()); }
 
+    const object& get_type() { return type; } const
+    const object& get_value() { return value; } const
+    const object& get_trace() { return trace; } const
+
 private:
     object type, value, trace;
 };