E0426 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0426.rs

E0426 needs a span_label, updating it from:

error[E0426]: use of undeclared label `'a`
  --> src/test/compile-fail/E0426.rs:13:15
   |
13 |         break 'a; //~ ERROR E0426
   |               ^^

To:

error[E0426]: use of undeclared label `'a`
  --> src/test/compile-fail/E0426.rs:13:15
   |
13 |         break 'a; //~ ERROR E0426
   |               ^^ undeclared label