cli: highlight error source headers as well

Highlighting "{n}: " will help to follow error sources containing multi-line
messages. I'm going to make revset/template alias errors be formatted as plain
error chain.

cli/src/command_error.rs

@@ -543,15 +543,20 @@ fn print_error_sources(ui: &Ui, source: Option<&dyn error::Error>) -> io::Result
     let Some(err) = source else {
         return Ok(());
     };
-    if err.source().is_none() {
-        writeln!(ui.stderr(), "Caused by: {err}")?;
-    } else {
-        writeln!(ui.stderr(), "Caused by:")?;
-        for (i, err) in iter::successors(Some(err), |err| err.source()).enumerate() {
-            writeln!(ui.stderr(), "{n}: {err}", n = i + 1)?;
-        }
-    }
-    Ok(())
+    ui.stderr_formatter()
+        .with_label("error_source", |formatter| {
+            if err.source().is_none() {
+                write!(formatter.labeled("heading"), "Caused by: ")?;
+                writeln!(formatter, "{err}")?;
+            } else {
+                writeln!(formatter.labeled("heading"), "Caused by:")?;
+                for (i, err) in iter::successors(Some(err), |err| err.source()).enumerate() {
+                    write!(formatter.labeled("heading"), "{}: ", i + 1)?;
+                    writeln!(formatter, "{err}")?;
+                }
+            }
+            Ok(())
+        })
 }
 
 fn handle_clap_error(ui: &mut Ui, err: &clap::Error, hints: &[String]) -> io::Result<ExitCode> {

cli/src/config/colors.toml

@@ -1,8 +1,10 @@
 [colors]
 "error" = { fg = "default", bold = true }
+"error_source" = { fg = "default" }
 "warning" = { fg = "default", bold = true }
 "hint" = { fg = "default" }
 "error heading" = { fg = "red", bold = true }
+"error_source heading" = { bold = true }
 "warning heading" = { fg = "yellow", bold = true }
 "hint heading" = { fg = "cyan", bold = true }
 

cli/tests/test_global_opts.rs

@@ -400,6 +400,13 @@ fn test_color_ui_messages() {
     Error: There is no jj repo in "."
     "###);
 
+    // error source
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["log", ".."]);
+    insta::assert_snapshot!(stderr.replace('\\', "/"), @r###"
+    Error: Path ".." is not in the repo "."
+    Caused by: Invalid component ".." in repo-relative path "../"
+    "###);
+
     // warning
     let (_stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["log", "@"]);
     insta::assert_snapshot!(stderr, @r###"
@@ -418,7 +425,7 @@ fn test_color_ui_messages() {
         ],
     );
     insta::assert_snapshot!(stdout, @r###"
-    [38;5;4m8bb159bc30a9859930e567eb9238a7c43ee6744d[39m
+    [38;5;4m167f90e7600a50f85c4f909b53eaf546faa82879[39m
     <Error: No commit available>  (elided revisions)
     0000000000000000000000000000000000000000
     "###);