[ER] Better error message for wrongly written break label

[leonardo-m]

This is an enhancement request. This code is wrong because it lacks a ':

fn main() {
    'LOOP: for _ in 0 .. 9 {
        break LOOP;
    }
}

It gives the error messages (rustc 1.50.0-nightly 1c389ff 2020-11-24):

error[E0425]: cannot find value `LOOP` in this scope
 --> ...test.rs:3:15
  |
3 |         break LOOP;
  |               ^^^^ not found in this scope

warning: unused label
 --> ...test.rs:2:5
  |
2 |     'LOOP: for _ in 0 .. 9 {
  |     ^^^^^
  |
  = note: `#[warn(unused_labels)]` on by default

error[E0571]: `break` with value from a `for` loop
 --> ...test.rs:3:9
  |
3 |         break LOOP;
  |         ^^^^^^^^^^ can only break with a value inside `loop` or breakable block
  |
help: instead, use `break` on its own without a value inside this `for` loop
  |
3 |         break;
  |         ^^^^^

error: aborting due to 2 previous errors; 1 warning emitted

But in such case I think it's better for the compiler to recognize in the error messages that the problem is probably just a lack of ' instead.

[sasurau4]

I'll try it.
@rustbot claim