chore(cr): more user-friendly error for valid configurable loaders

Author: Kocal

lib/WebpackConfig.js

@@ -721,7 +721,7 @@ class WebpackConfig {
 
     configureLoaderRule(name, callback) {
         if (!(name in this.loaderConfigurationCallbacks)) {
-            throw new Error(`Loader "${name}" is not configurable. Either open an issue or a pull request.`);
+            throw new Error(`Loader "${name}" is not configurable. Valid loaders are "${Object.keys(this.loaderConfigurationCallbacks).join('", "')}".`);
         }
 
         if (typeof callback !== 'function') {

test/WebpackConfig.js

@@ -1161,7 +1161,7 @@ describe('WebpackConfig object', () => {
 
             expect(() => {
                 config.configureLoaderRule('vue');
-            }).to.throw('Loader "vue" is not configurable. Either open an issue or a pull request.');
+            }).to.throw('Loader "vue" is not configurable. Valid loaders are "eslint".');
         });
 
         it('Call method with not a valid callback', () => {