Update: Improve error reporting when extending missing config (fixes …

lib/config/config-file.js

@@ -390,6 +390,13 @@ function applyExtends(config, filePath, relativeTo) {
             debug(`Loading ${parentPath}`);
             return ConfigOps.merge(load(parentPath, false, relativeTo), previousValue);
         } catch (e) {
+            if (parentPath.indexOf("plugin:") === 0 || parentPath.indexOf("eslint:") === 0) {
+                e.message = `Failed to load config "${parentPath}" to extend from.`;
+                e.messageTemplate = "extend-config-missing";
+                e.messageData = {
+                    configName: parentPath
+                };
+            }
 
             /*
              * If the file referenced by `extends` failed to load, add the path

messages/extend-config-missing.txt

@@ -0,0 +1,3 @@
+ESLint couldn't find the config "<%- configName %>" to extend from. Please check that the name of the config is correct.
+
+If you still have problems, please stop by https://gitter.im/eslint/eslint to chat with the team.

tests/lib/config/config-file.js

@@ -199,7 +199,7 @@ describe("ConfigFile", () => {
 
         });
 
-        it("should throw an error when extends config is not found", () => {
+        it("should throw an error when extends config module is not found", () => {
 
             const configDeps = {
                 "../util/module-resolver": createStubModuleResolver({})
@@ -216,6 +216,40 @@ describe("ConfigFile", () => {
 
         });
 
+        it("should throw an error when an eslint config is not found", () => {
+
+            const configDeps = {
+                "../util/module-resolver": createStubModuleResolver({})
+            };
+
+            const StubbedConfigFile = proxyquire("../../../lib/config/config-file", configDeps);
+
+            assert.throws(() => {
+                StubbedConfigFile.applyExtends({
+                    extends: "eslint:foo",
+                    rules: { eqeqeq: 2 }
+                }, "/whatever");
+            }, /Failed to load config "eslint:foo" to extend from./);
+
+        });
+
+        it("should throw an error when a plugin config is not found", () => {
+
+            const configDeps = {
+                "../util/module-resolver": createStubModuleResolver({})
+            };
+
+            const StubbedConfigFile = proxyquire("../../../lib/config/config-file", configDeps);
+
+            assert.throws(() => {
+                StubbedConfigFile.applyExtends({
+                    extends: "plugin:foo",
+                    rules: { eqeqeq: 2 }
+                }, "/whatever");
+            }, /Failed to load config "plugin:foo" to extend from./);
+
+        });
+
         it("should apply extensions recursively when specified from package", () => {
 
             const resolvedPaths = [