refactor(pass-style): better non-remotable function error (#2267)

closes: #XXXX
refs: Agoric/agoric-sdk#9097

## Description

As the changed test case demonstrates, before this PR, testing the
`passStyleOf` a normal unmarked function was correctly rejected, but
with a terrible error message, such as:
- `"[Symbol(passStyle)]" property expected: "[Function ...]"`

This change aligns with the error message already used for an unmarked
objects-as-closure object. For an unmarked function, the error would be
- `Remotables must be explicitly declared: "[Function ...]"`

### Security Considerations

none

### Scaling Considerations

none

### Documentation Considerations

none

### Testing & Compatibility Considerations

There might be tests with golden text based on the old error message.
These tests would need to be revised to the new text. During the
transition, golden text error tests should use a regexp to be compat
with both the before and after of this PR, with a link to the PR itself
so we know when we can clean it up.

I found three such dependencies. One in endo, which this PR fixes. None
in agoric-sdk master. Two in
Agoric/agoric-sdk#9097 , both of which have now
been revised to be compat before and after.


### Upgrade Considerations

none

- ~[ ] Includes `*BREAKING*:` in the commit message with migration
instructions for any breaking change.~
- ~[ ] Updates `NEWS.md` for user-facing changes.~

packages/exo/test/test-heap-classes.js

@@ -180,12 +180,13 @@ test('missing interface', t => {
   );
   const greeterMaker = makeExo('greeterMaker', undefined, {
     makeSayHello() {
-      return () => 'hello';
+      const helloFunc = () => 'hello';
+      return harden(helloFunc);
     },
   });
   t.throws(() => greeterMaker.makeSayHello(), {
     message:
-      'In "makeSayHello" method of (greeterMaker): result: "[Symbol(passStyle)]" property expected: "[Function <anon>]"',
+      'In "makeSayHello" method of (greeterMaker): result: Remotables must be explicitly declared: "[Function helloFunc]"',
   });
   t.is(greeterMaker[GET_INTERFACE_GUARD]?.(), undefined);
 });

packages/pass-style/src/remotable.js

@@ -87,7 +87,11 @@ const checkRemotableProtoOf = (original, check) => {
   //        }}
   //
   const proto = getPrototypeOf(original);
-  if (proto === objectPrototype || proto === null) {
+  if (
+    proto === objectPrototype ||
+    proto === null ||
+    proto === Function.prototype
+  ) {
     return (
       reject && reject`Remotables must be explicitly declared: ${q(original)}`
     );