Avoid synchronous error.

src/module.js

@@ -47,14 +47,15 @@ function module_variable(observer) {
   return new Variable(TYPE_NORMAL, this, observer);
 }
 
-function module_value(name) {
+async function module_value(name) {
   var v = this._scope.get(name);
   if (!v) throw new RuntimeError(name + " is not defined");
   if (v._observer === no_observer) {
     v._observer = true;
     this._runtime._dirty.add(v);
   }
-  return this._runtime._compute().then(() => v._promise);
+  await this._runtime._compute();
+  return v._promise;
 }
 
 function module_derive(injects, injectModule) {

test/module/value-test.js

@@ -62,6 +62,18 @@ tape("module.value(name) supports missing variables", async test => {
   }
 });
 
+tape("module.value(name) returns a promise on error", async test => {
+  const runtime = new Runtime();
+  const module = runtime.module();
+  const promise = module.value("bar");
+  try {
+    await promise;
+    test.fail();
+  } catch (error) {
+    test.deepEqual(error.message, "bar is not defined");
+  }
+});
+
 tape("module.value(name) does not force recomputation", async test => {
   let foo = 0;
   const runtime = new Runtime();