fix(frontmatter): Be more direct in unclosed error

For comparison, rustc's message is:

> error: unclosed frontmatter
>   --> $DIR/frontmatter-whitespace-2.rs:1:1
>    |
> LL | / ---cargo
> ...  |
> LL | |
>    | |_^
>    |
> note: frontmatter opening here was not closed
>   --> $DIR/frontmatter-whitespace-2.rs:1:1
>    |
> LL | ---cargo
>    | ^^^

Author: epage

src/cargo/util/frontmatter.rs

@@ -80,7 +80,7 @@ impl<'s> ScriptSource<'s> {
         source.open = Some(open_start..open_end);
         let Some(info_nl) = input.find_slice("\n") else {
             return Err(FrontmatterError::new(
-                format!("no closing `{fence_pattern}` found for frontmatter"),
+                format!("unclosed frontmatter; expected `{fence_pattern}`"),
                 open_start..open_end,
             ));
         };
@@ -109,7 +109,7 @@ impl<'s> ScriptSource<'s> {
                 ));
             }
             return Err(FrontmatterError::new(
-                format!("no closing `{fence_pattern}` found for frontmatter"),
+                format!("unclosed frontmatter; expected `{fence_pattern}`"),
                 open_start..open_end,
             ));
         };
@@ -636,7 +636,7 @@ time="0.1.25"
 fn main() {}
 "#,
             ),
-            str!["no closing `---` found for frontmatter"],
+            str!["unclosed frontmatter; expected `---`"],
         );
     }
 }

tests/testsuite/script/rustc_fixtures/frontmatter-whitespace-2.stderr

@@ -1,4 +1,4 @@
-[ERROR] no closing `---` found for frontmatter
+[ERROR] unclosed frontmatter; expected `---`
  --> script:1:1
   |
 1 | ---cargo

tests/testsuite/script/rustc_fixtures/unclosed-1.stderr

@@ -1,4 +1,4 @@
-[ERROR] no closing `----` found for frontmatter
+[ERROR] unclosed frontmatter; expected `----`
  --> script:1:1
   |
 1 | ----cargo

tests/testsuite/script/rustc_fixtures/unclosed-2.stderr

@@ -1,4 +1,4 @@
-[ERROR] no closing `----` found for frontmatter
+[ERROR] unclosed frontmatter; expected `----`
  --> script:1:1
   |
 1 | ----cargo

tests/testsuite/script/rustc_fixtures/unclosed-3.stderr

@@ -1,4 +1,4 @@
-[ERROR] no closing `----` found for frontmatter
+[ERROR] unclosed frontmatter; expected `----`
  --> script:1:1
   |
 1 | ----cargo

tests/testsuite/script/rustc_fixtures/unclosed-4.stderr

@@ -1,4 +1,4 @@
-[ERROR] no closing `----` found for frontmatter
+[ERROR] unclosed frontmatter; expected `----`
  --> script:1:1
   |
 1 | ----cargo

tests/testsuite/script/rustc_fixtures/unclosed-5.stderr

@@ -1,4 +1,4 @@
-[ERROR] no closing `----` found for frontmatter
+[ERROR] unclosed frontmatter; expected `----`
  --> script:1:1
   |
 1 | ----cargo