The following code demonstrates this behavior. As foo.on_error is
'throw_last' the on_error_1 handler is executed as well as on_error_2
which is the exception throwed back to the bar handler.

from circuits import Event, Component, handler, Debugger
import pytest

class foo(Event):
	on_error = 'throw_last'

class bar(Event):
	pass

class Foo(Component):
	@handler('foo', priority=2)
	def _on_error_1(self):
		raise ValueError('test')

	@handler('foo', priority=1)
	def _on_error_2(self):
		raise ValueError('test2')

	def bar(self):
		with pytest.raises(ValueError) as exc:
			x = yield self.call(foo())
		assert exc.value.args[0] == 'test2'
		print('DONE')

x = Foo()
x += Debugger()
x.fire(bar())
x.run()