E0178 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0178.rs

E0178 needs a span_label, updating it from:

error[E0178]: expected a path on the left-hand side of `+`, not `&'a Foo`
  --> src/test/compile-fail/E0178.rs:14:8
   |
14 |     w: &'a Foo + Copy, //~ ERROR E0178
   |        ^^^^^^^
   |
help: try adding parentheses (per RFC 438):
   |     w: &'a (Foo + Copy), //~ ERROR E0178

To:

error[E0178]: expected a path on the left-hand side of `+`, not `&'a Foo`
  --> src/test/compile-fail/E0178.rs:14:8
   |
14 |     w: &'a Foo + Copy, //~ ERROR E0178
   |        ^^^^^^^ expected a path
   |
help: try adding parentheses (per RFC 438):
   |     w: &'a (Foo + Copy), //~ ERROR E0178

[munyari]

I'll take this