def construct_error_generator_middleware(error_generators):
    """
    Constructs a middleware which intercepts requests for any method found in
    the provided mapping of endpoints to generator functions, returning
    whatever error message the generator function returns.  Callbacks must be
    functions with the signature `fn(method, params)`.
    """
    def error_generator_middleware(make_request, web3):
        def middleware(method, params):
            if method in error_generators:
                error_msg = error_generators[method](method, params)
                return {'error': error_msg}
            else:
                return make_request(method, params)
        return middleware
    return error_generator_middleware