Add exception hierarchy to the documentation.

docs/extensions/exception_hierarchy.py

@@ -0,0 +1,27 @@
+from docutils.parsers.rst import Directive
+from docutils.parsers.rst import states, directives
+from docutils.parsers.rst.roles import set_classes
+from docutils import nodes
+from sphinx.locale import _
+
+class exception_hierarchy(nodes.General, nodes.Element):
+    pass
+
+def visit_exception_hierarchy_node(self, node):
+    self.body.append(self.starttag(node, 'div', CLASS='exception-hierarchy-content'))
+
+def depart_exception_hierarchy_node(self, node):
+    self.body.append('</div>\n')
+
+class ExceptionHierarchyDirective(Directive):
+    has_content = True
+
+    def run(self):
+        self.assert_has_content()
+        node = exception_hierarchy('\n'.join(self.content))
+        self.state.nested_parse(self.content, self.content_offset, node)
+        return [node]
+
+def setup(app):
+    app.add_node(exception_hierarchy, html=(visit_exception_hierarchy_node, depart_exception_hierarchy_node))
+    app.add_directive('exception_hierarchy', ExceptionHierarchyDirective)