Fix confusing error with local feature (#834)

* obtuse and confusing error with local feature under WSL #834

* addressed review comments and modified error message accordingly

* review comments addressed and error message changed as needed

* error message tab removed and variable moved to if condition

Author: prathameshzarkar9

src/spec-configuration/containerFeaturesConfiguration.ts

@@ -698,8 +698,12 @@ export async function getFeatureIdType(params: CommonParams, userFeatureId: stri
 
 	// Legacy feature-set ID
 	if (!userFeatureId.includes('/') && !userFeatureId.includes('\\')) {
-		output.write(`Legacy feature '${userFeatureId}' not supported. Please check https://containers.dev/features for replacements.`, LogLevel.Error);
-		throw new ContainerError({ description: `Legacy feature '${userFeatureId}' not supported. Please check https://containers.dev/features for replacements.` });
+		const errorMessage = `Legacy feature '${userFeatureId}' not supported. Please check https://containers.dev/features for replacements.
+If you were hoping to use local Features, remember to prepend your Feature name with "./". Please check https://containers.dev/implementors/features-distribution/#addendum-locally-referenced for more information.`;
+		output.write(errorMessage, LogLevel.Error);
+		throw new ContainerError({
+			description: errorMessage
+		});
 	}
 
 	// Direct tarball reference

src/test/cli.build.test.ts

@@ -27,6 +27,17 @@ describe('Dev Containers CLI', function () {
 
 	describe('Command build', () => {
 
+		it('should fail to build with correct error message for local feature', async () => {
+			const testFolder = `${__dirname}/configs/image-with-local-feature`;
+			try {
+				await shellExec(`${cli} build --workspace-folder ${testFolder} --image-name demo:v1`);
+			} catch (error) {
+				const res = JSON.parse(error.stdout);
+				assert.equal(res.outcome, 'error');
+				assert.match(res.message, /prepend your Feature name with/);
+			}
+		});
+
 		it('should correctly configure the image name to push from --image-name with --push true', async () => {
 			const testFolder = `${__dirname}/configs/example`;
 			try {

src/test/configs/image-with-local-feature/.devcontainer.json

@@ -0,0 +1,6 @@
+{
+	"image": "ubuntu:latest",
+	"features": {
+		"myfeature": {}
+	}
+}