core: lock stdout before printing an error message to stderr

Adds a new eprintln_locked macro which locks STDOUT before logging
to STDERR. This patch also replaces instances of eprintln with
eprintln_locked to avoid interleaving lines.

Fixes #1941, Closes #1968

crates/core/main.rs

@@ -47,7 +47,7 @@ type Result<T> = ::std::result::Result<T, Box<dyn error::Error>>;
 
 fn main() {
     if let Err(err) = Args::parse().and_then(try_main) {
-        eprintln!("{}", err);
+        eprintln_locked!("{}", err);
         process::exit(2);
     }
 }