compiletest/runtest: format ErrorKind with Display

The strings are nouns for the most part, so we give ErrorKind::Help a
more sensible string. This reduces quote hiccups in failure output.

unexpected "error": '...'
↓
unexpected error: '...'

Author: hdhoang

src/tools/compiletest/src/errors.rs

@@ -45,7 +45,7 @@ impl FromStr for ErrorKind {
 impl fmt::Display for ErrorKind {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
-            ErrorKind::Help => write!(f, "help"),
+            ErrorKind::Help => write!(f, "help message"),
             ErrorKind::Error => write!(f, "error"),
             ErrorKind::Note => write!(f, "note"),
             ErrorKind::Suggestion => write!(f, "suggestion"),

src/tools/compiletest/src/runtest.rs

@@ -1040,7 +1040,7 @@ actual:\n\
                 None => {
                     if self.is_unexpected_compiler_message(actual_error, expect_help, expect_note) {
                         self.error(
-                            &format!("{}:{}: unexpected {:?}: '{}'",
+                            &format!("{}:{}: unexpected {}: '{}'",
                                      file_name,
                                      actual_error.line_num,
                                      actual_error.kind.as_ref()