Rollup merge of rust-lang#23332 - jakub-:rustup-curl-silent-flag, r=brson

curl's progress meter would otherwise interfere with sudo's password prompt.

In addition, add the -f flag to make sure 4xx status codes are treated as errors.

r? @brson

Author: Manishearth

src/doc/trpl/installing-rust.md

@@ -6,14 +6,14 @@ Linux or a Mac, all you need to do is this (note that you don't need to type
 in the `$`s, they just indicate the start of each command):
 
 ```bash
-$ curl -L https://static.rust-lang.org/rustup.sh | sudo sh
+$ curl -sf -L https://static.rust-lang.org/rustup.sh | sudo sh
 ```
 
 If you're concerned about the [potential insecurity](http://curlpipesh.tumblr.com/) of using `curl | sudo sh`,
 please keep reading and see our disclaimer below. And feel free to use a two-step version of the installation and examine our installation script:
 
 ```bash
-$ curl -L https://static.rust-lang.org/rustup.sh -O
+$ curl -f -L https://static.rust-lang.org/rustup.sh -O
 $ sudo sh rustup.sh
 ```