API functions returning Promises should not throw exceptions.

Source/bindings/v8/ExceptionState.h

Index: Source/bindings/v8/ExceptionState.h
diff --git a/Source/bindings/v8/ExceptionState.h b/Source/bindings/v8/ExceptionState.h
index d9c2b05b8efbf964243352d5fbea2f0cb792aa08..e09e223f8ea44d7d7a4b8fd5aaa7e585eaffe7bd 100644
--- a/Source/bindings/v8/ExceptionState.h
+++ b/Source/bindings/v8/ExceptionState.h
@@ -32,6 +32,7 @@
 #define ExceptionState_h
 
 #include "bindings/v8/ScopedPersistent.h"
+#include "bindings/v8/ScriptPromise.h"
 #include "bindings/v8/V8ThrowException.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/text/WTFString.h"
@@ -92,6 +93,13 @@ public:
         return true;
     }
 
+    ScriptPromise rejectedPromise()

[haraken, 2014/04/10 12:09:17]

Shall we just call it "reject", just like other reject methods in
ScriptPromise.h?

[yhirano, 2014/04/11 10:19:04]

Done.

+    {
+        if (hadException())
+            return ScriptPromise::reject(m_exception.newLocal(m_isolate), m_isolate);
+        return ScriptPromise::reject(V8ThrowException::createError(v8GeneralError, "Unknown Error", m_isolate), m_isolate);
+    }
+
     Context context() const { return m_context; }
     const char* propertyName() const { return m_propertyName; }
     const char* interfaceName() const { return m_interfaceName; }