Restrict error code length to 80 columns

The global restriction is 100, but since error codes are printed out via
--explain we want to restrict them to just 80 columns.

src/libsyntax_ext/error_codes.rs

@@ -1,9 +1,10 @@
 use syntax::register_long_diagnostics;
 
 // Error messages for EXXXX errors.
-// Each message should start and end with a new line, and be wrapped to 80 characters.
-// In vim you can `:set tw=80` and use `gq` to wrap paragraphs. Use `:set tw=0` to disable.
-register_long_diagnostics! {
+// Each message should start and end with a new line, and be wrapped to 80
+// characters.  In vim you can `:set tw=80` and use `gq` to wrap paragraphs. Use
+// `:set tw=0` to disable.
+syntax::register_diagnostics! {
 E0660: r##"
 The argument to the `asm` macro is not well-formed.