Rollup merge of rust-lang#37066 - nrc:stderr, r=alexcrichton

Error monitor should emit error to stderr instead of stdout

We are pretty consistent about emitting to stderr, except for when there is actually an error, in which case we emit to stdout. This seems a bit backwards. This PR just changes that exception to emit to stderr. This is useful for the RLS since the LS protocol uses stdout (grrr).

r? @alexcrichton

src/librustc_driver/lib.rs

@@ -1110,7 +1110,7 @@ pub fn monitor<F: FnOnce() + Send + 'static>(f: F) {
                              errors::Level::Note);
             }
 
-            println!("{}", str::from_utf8(&data.lock().unwrap()).unwrap());
+            writeln!(io::stderr(), "{}", str::from_utf8(&data.lock().unwrap()).unwrap()).unwrap();
         }
 
         exit_on_err();