lint: introduce LintError; count diagnostic errors

src/main.rs

@@ -38,6 +38,12 @@ enum FormatterError {
     Io(#[from] io::Error),
 }
 
+#[derive(Error, Debug)]
+enum LintError {
+    #[error("Lint errors found in {file_count} file(s)")]
+    DiagnosticErrors { file_count: usize },
+}
+
 #[derive(Parser, Debug, Clone)]
 #[command(
     name = "nff",
@@ -337,7 +343,7 @@ fn process_lint_command(
     };
 
     let is_multiple_files = files.len() > 1;
-    let mut has_errors = false;
+    let mut error_file_count = 0;
 
     for file_path in files {
         if let Err(e) = process_single_file_lint(
@@ -352,16 +358,18 @@ fn process_lint_command(
             is_multiple_files,
         ) {
             eprintln!("Error processing {}: {}", file_path, e);
-            has_errors = true;
+            error_file_count += 1;
             if !is_multiple_files {
                 return Err(e);
             }
         }
     }
 
-    // Exit with non-zero code if any file had errors
-    if has_errors {
-        std::process::exit(1);
+    if error_file_count > 0 {
+        return Err(LintError::DiagnosticErrors {
+            file_count: error_file_count,
+        }
+        .into());
     }
 
     Ok(())