E0001 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0001.rs

We need to add a span_label to update E0001 from:

error[E0001]: unreachable pattern
  --> src/test/compile-fail/E0001.rs:16:9
   |
16 |         _ => {/* ... */} //~ ERROR E0001
   |         ^

To:

error[E0001]: unreachable pattern
  --> src/test/compile-fail/E0001.rs:16:9
   |
16 |         _ => {/* ... */} //~ ERROR E0001
   |         ^ unreachable pattern

[saml]

Taking it