fix(cli): warn on unknown commands

Emit a warning and exit with status code 1.

Based on tj/commander.js#1088 (comment)

packages/api/cli/src/electron-forge.ts

@@ -34,7 +34,13 @@ program
   .command('make', 'Generate distributables for the current Electron application')
   .command('start', 'Start the current Electron application')
   .command('publish', 'Publish the current Electron application to GitHub')
-  .command('install', 'Install an Electron application from GitHub');
+  .command('install', 'Install an Electron application from GitHub')
+  .on('command:*', () => {
+    console.error();
+    console.error(`Unknown command "${program.args.join(' ')}".`.red);
+    console.error('See --help for a list of available commands.');
+    process.exit(1);
+  });
 
 (async () => {
   let goodSystem;

packages/api/cli/test/cli_spec.ts

@@ -0,0 +1,14 @@
+import chai, { expect } from 'chai';
+import chaiAsPromised from 'chai-as-promised';
+import path from 'path';
+import spawnPromise from 'cross-spawn-promise';
+
+chai.use(chaiAsPromised);
+
+describe('cli', () => {
+  it('should fail on unknown subcommands', async () => {
+    const error = await expect(spawnPromise(path.resolve(__dirname, '../../../../node_modules/.bin/ts-node'), [path.resolve(__dirname, '../src/electron-forge.ts'), 'nonexistent'])).to.eventually.be.rejected;
+    expect(error.exitStatus).to.equal(1);
+    expect(error.stderr.toString()).to.match(/Unknown command "nonexistent"/);
+  });
+});