fix(bundler): catch and warn about missing credentials

lib/manager/bundler/artifacts.js

@@ -154,9 +154,15 @@ async function getArtifacts(
     ) {
       throw new Error('bundler-fs');
     }
+    if (
+      err.stdout &&
+      err.stdout.includes('Please supply credentials for this source')
+    ) {
+      throw new Error('bundler-credentials');
+    }
     logger.info(
       { err, message: err.message },
-      'Failed to generate bundler.lock (unknown error)'
+      'Failed to generate Gemfile.lock (unknown error)'
     );
     throw new Error('bundler-unknown');
   }

lib/workers/branch/index.js

@@ -327,6 +327,10 @@ async function processBranch(branchConfig, prHourlyLimitReached, packageFiles) {
       logger.warn(
         'It is necessary to run Renovate in gitFs mode - contact your bot administrator'
       );
+    } else if (err.message === 'bundler-credentials') {
+      logger.warn(
+        'Bundler credentials are necessary in order to generate Gemfile.lock updates'
+      );
     } else if (err.message === 'bundler-unknown') {
       logger.warn('Unknown bundler error');
     } else if (