Remind the user of the hint option

Suggestion from AbdouSeck https://github.com/rust-lang/rustlings/issues/424#issuecomment-639870331
for when the student's code has errors.

src/verify.rs

@@ -54,6 +54,7 @@ fn compile_and_run_interactively(exercise: &Exercise) -> Result<bool, ()> {
         Err(output) => {
             warn!("Ran {} with errors", exercise);
             println!("{}", output.stdout);
+            println!("{}", output.stderr);
             return Err(());
         }
     };