Remove the error tuple return value for middlewares

It wasn't interesting compared to simply returning a halt tuple
with an explicit reply.

doc/src/manual/cowboy_middleware.ezdoc

@@ -22,7 +22,6 @@ optionally with its contents modified.
 	-> {ok, Req, Env}
 	| {suspend, Module, Function, Args}
 	| {halt, Req}
-	| {error, StatusCode, Req}
 
 Types:
 
@@ -31,7 +30,6 @@ Types:
 * Module = module()
 * Function = atom()
 * Args = [any()]
-* StatusCode = cowboy:http_status()
 
 Execute the middleware.
 
@@ -47,8 +45,3 @@ The `halt` return value stops Cowboy from doing any further
 processing of the request, even if there are middlewares
 that haven't been executed yet. The connection may be left
 open to receive more requests from the client.
-
-The `error` return value sends an error response identified
-by the `StatusCode` and then proceeds to terminate the
-connection. Middlewares that haven't been executed yet
-will not be called.