Improve error handling

bin/handlebars.mjs

@@ -125,6 +125,12 @@ Precompiler.loadTemplates(argv, function (err, opts) {
   if (opts.help || (!opts.templates.length && !opts.version)) {
     parser.showHelp('log');
   } else {
-    Precompiler.cli(opts);
+    // cli() is async (returns a Promise), so errors would become unhandled
+    // rejections. Re-throw via nextTick to surface them as uncaught exceptions.
+    Promise.resolve(Precompiler.cli(opts)).catch((error) => {
+      process.nextTick(() => {
+        throw error;
+      });
+    });
   }
 });

tasks/tests/cli.test.js

@@ -1,4 +1,5 @@
 const fs = require('fs');
+const { exec } = require('child_process');
 const { execCommand, FileTestHelper } = require('cli-testlab');
 const Handlebars = require('../../lib');
 
@@ -245,6 +246,21 @@ describe('bin/handlebars', function () {
     });
   });
 
+  describe('error handling', function () {
+    it('should not produce unhandled promise rejections on async errors', async function () {
+      const { stderr } = await new Promise((resolve) => {
+        exec(
+          `node --unhandled-rejections=warn ./bin/handlebars.mjs -i "<div>test</div>" -N test --map /nonexistent/dir/test.map`,
+          (error, stdout, stderr) => {
+            resolve({ exitCode: error ? error.code : 0, stderr });
+          }
+        );
+      });
+
+      expect(stderr).not.toMatch(/unhandled/i);
+    });
+  });
+
   describe('negated boolean flags', function () {
     it('--no-amd negates --amd (issue #1673)', async function () {
       const result = await execCommand(