Improve CLI error message when no files are specified

src/tslint-cli.ts

@@ -232,7 +232,7 @@ if (parsed.unknown.length !== 0) {
 const argv = commander.opts() as any as Argv;
 
 if (!(argv.init || argv.test !== undefined || argv.project !== undefined || commander.args.length > 0)) {
-    console.error("Missing files");
+    console.error("No files specified. Use --project to lint a project folder.");
     process.exit(1);
 }
 

test/executable/executableTests.ts

@@ -37,7 +37,9 @@ describe("Executable", function(this: Mocha.ISuiteCallbackContext) {
                 assert.isNotNull(err, "process should exit with error");
                 assert.strictEqual(err.code, 1, "error code should be 1");
 
-                assert.include(stderr, "Missing files", "stderr should contain notification about missing files");
+                assert.include(stderr,
+                    "No files specified. Use --project to lint a project folder.",
+                    "stderr should contain notification about missing files");
                 assert.strictEqual(stdout, "", "shouldn't contain any output in stdout");
                 done();
             });