Throw V8 exception on invalid Sass_Value

In the unlikely event libsass gives us the
value we don't understand use V8 exception
to report failure and exit before calling
a user-defined custom function.

XXX We do not have currently any way
    to test this (that would require mocking
    libsass).

src/callback_bridge.h

@@ -98,7 +98,12 @@ T CallbackBridge<T, L>::operator()(std::vector<void*> argv) {
      * post_process_args().
      */
     Nan::HandleScope scope;
+    Nan::TryCatch try_catch;
     std::vector<v8::Local<v8::Value>> argv_v8 = pre_process_args(argv);
+    if (try_catch.HasCaught()) {
+      Nan::FatalException(try_catch);
+    }
+
     argv_v8.push_back(Nan::New(wrapper));
 
     return this->post_process_return_value(
@@ -149,6 +154,9 @@ void CallbackBridge<T, L>::dispatched_async_uv_callback(uv_async_t *req) {
   Nan::TryCatch try_catch;
 
   std::vector<v8::Local<v8::Value>> argv_v8 = bridge->pre_process_args(bridge->argv);
+  if (try_catch.HasCaught()) {
+    Nan::FatalException(try_catch);
+  }
   argv_v8.push_back(Nan::New(bridge->wrapper));
 
   bridge->callback->Call(argv_v8.size(), &argv_v8[0]);

src/sass_types/factory.cpp

@@ -39,7 +39,9 @@ namespace SassTypes
       return new Error(v);
 
     default:
-      throw std::invalid_argument("Unknown type encountered.");
+      const char *msg = "Unknown type encountered.";
+      Nan::ThrowTypeError(Nan::New<v8::String>(msg).ToLocalChecked());
+      return new Error(sass_make_error(msg));
     }
   }