API functions returning Promises should not throw exceptions.

Source/bindings/scripts/v8_types.py

Index: Source/bindings/scripts/v8_types.py
diff --git a/Source/bindings/scripts/v8_types.py b/Source/bindings/scripts/v8_types.py
index c2ebdcd17f1ec7428f7727a8f514f7d577c96930..1ca7631c8ef85a9400fc9bfc4c11ac834d263bf9 100644
--- a/Source/bindings/scripts/v8_types.py
+++ b/Source/bindings/scripts/v8_types.py
@@ -523,7 +523,7 @@ def v8_value_to_cpp_value_array_or_sequence(native_array_element_type, v8_value,
     return expression
 
 
-def v8_value_to_local_cpp_value(idl_type, extended_attributes, v8_value, variable_name, index=None, declare_variable=True, isolate='info.GetIsolate()', used_in_private_script=False):
+def v8_value_to_local_cpp_value(idl_type, extended_attributes, v8_value, variable_name, index=None, declare_variable=True, isolate='info.GetIsolate()', used_in_private_script=False, return_promise=False):
     """Returns an expression that converts a V8 value to a C++ value and stores it as a local value."""
 
     # FIXME: Support union type.
@@ -550,6 +550,12 @@ def v8_value_to_local_cpp_value(idl_type, extended_attributes, v8_value, variabl
     # v8::TryCatch.
     suffix = ''
 
+    if return_promise:
+        suffix += '_PROMISE'
+        args.append('info')
+        if macro == 'TONATIVE_VOID_EXCEPTIONSTATE':
+            args.append('ScriptState::current(%s)' % isolate)
+
     if declare_variable:
         args.insert(0, this_cpp_type)
     else: