Add a non-decorator version of the error handler register function

The main application object has a register_error_handler function
which mirrors the decorator's functionality. According to the principle
of least surprise, make sure blueprints also have this convenience function.

flask/blueprints.py

@@ -399,3 +399,14 @@ class Blueprint(_PackageBoundObject):
                 self.name, code_or_exception, f))
             return f
         return decorator
+
+    def register_error_handler(self, code_or_exception, f):
+        """Non-decorator version of the :meth:`errorhandler` error attach
+        function, akin to the :meth:`~flask.Flask.register_error_handler`
+        application-wide function of the :class:`~flask.Flask` object but
+        for error handlers limited to this blueprint.
+
+        .. versionadded:: 0.11
+        """
+        self.record_once(lambda s: s.app._register_error_handler(
+            self.name, code_or_exception, f))