Custom python exception for printing multiple exceptions

MultiErrorError.py

class MultiErrorError(Exception):
    '''Gather multiple errors and include all of their info in one exception'''
    def __init__(self, msg, errors):
        '''Create a MultiErrorError

        Arguments:
        msg -- text to show in top-level exception after "N errors raised while "
        errors -- list of error tuples from calls to sys.exc_info
        '''
        self.msg = msg
        self.errors = errors
    def __str__(self):
        base_msg = "%s errors raised while %s" % (len(self.errors), self.msg)
        from traceback import format_exception
        traces = map(lambda x: "".join(format_exception(*x)), self.errors)
        return base_msg + reduce(lambda x,y: "\n\n%s\n\n%s" % (x,y), traces)