Add information about parser/index to error messages when parsing configuration file content.

lib/markdownlint.js

@@ -896,12 +896,13 @@ function parseConfiguration(name, content, parsers) {
   let config = null;
   let message = "";
   const errors = [];
+  let index = 0;
   // Try each parser
   (parsers || [ JSON.parse ]).every((parser) => {
     try {
       config = parser(content);
     } catch (error) {
-      errors.push(error.message);
+      errors.push(`Parser ${index++}: ${error.message}`);
     }
     return !config;
   });