refactor(formatMessage): raise meaningful exception

* Adding spec for Antl.formatMessage

* Throwing exception on Antl.formatMessage when translation key is missing

src/Antl/index.js

@@ -108,9 +108,19 @@ class Antl {
 
   /**
    * @see('Formatter.formatMessage')
+   *
+   * @throws {InvalidArgumentException} If translation is not found
    */
   formatMessage (key, ...args) {
-    return this._formatter.formatMessage(this.get(key), ...args)
+    const rawMessage = this.get(key)
+
+    if (!rawMessage) {
+      throw GE
+        .InvalidArgumentException
+        .invalidParameter(`Missing ${this._locale} translation for key '${key}'`)
+    }
+
+    return this._formatter.formatMessage(rawMessage, ...args)
   }
 
   /**

test/antl.spec.js

@@ -45,6 +45,37 @@ test.group('Antl', () => {
     assert.equal(antl.formatRelative(today.getTime()), 'now')
   })
 
+  test('format message for a given locale', (assert) => {
+    const antl = new Antl('en-us', {
+      'en-us': {
+        'header': {
+          'hello': 'Hello {name}'
+        }
+      }
+    })
+
+    assert.equal(antl.formatMessage('header.hello', { name: 'Peter' }), 'Hello Peter')
+  })
+
+  test('format message throws exception when translation is missing', (assert) => {
+    const antl = new Antl('en-us', {
+      'en-us': {
+        'validations': {
+          'name.required': 'Name is required'
+        }
+      },
+      '*': {
+        'validations': {
+          'email.required': 'Email is required'
+        }
+      }
+    })
+
+    const fn = () => antl.formatMessage('validations.age.required')
+
+    assert.throw(fn, 'E_INVALID_PARAMETER: Missing en-us translation for key \'validations.age.required\'')
+  })
+
   test('get message for a key', (assert) => {
     const antl = new Antl('en-us', {
       'en-us': {