Fix assert.throws in node 8.9

Author: stasm

fluent/test/patterns_test.js

@@ -88,7 +88,7 @@ suite('Patterns', function(){
       const msg = bundle.getMessage('foo');
       assert.throws(
         () => bundle.formatPattern(msg.value, args, errs),
-        new TypeError("Invalid Pattern type")
+        /Invalid Pattern type/
       );
     });
 

fluent/test/values_format_test.js

@@ -63,7 +63,7 @@ suite('Formatting values', function(){
     const msg = bundle.getMessage('key5');
     assert.throws(
       () => bundle.formatPattern(msg.value, args, errs),
-      new TypeError("Invalid Pattern type")
+      /Invalid Pattern type/
     );
   });