gen-host-js: make ComponentError string payload non-enumerable

[guybedford]

This makes the payload property non-enumerable when it is a string, while keeping it enumerable when it is not a string. This way it doesn't show up twice in Node.js error stacks. Browser error logging remains identical both for thrown and logged error strings.

In addition, when the payload is not a string, we add an additional note - (see error.payload) to the error message so that the message will look like:

ComponentError: [object Object] (see error.payload)

for object errors, while remaining enumerable so that error logging environments that do log the enumerable payload property for non-strings will be able to log the object as well.

For example, here's wit-bindgen error strings before and after in Node.js.

Before:

file:///Users/gbedford/Projects/test/test.js:212
    throw new ComponentError(variant22.val);
          ^

ComponentError: failed to extract interface information from component

Caused by:
    magic header not detected: bad magic number (at offset 0x0)
    at generate (file:///Users/gbedford/Projects/test/test.js:212:11)
    at file:///Users/gbedford/Projects/test/test.js:28:34 {
  payload: 'failed to extract interface information from component\n' +
    '\n' +
    'Caused by:\n' +
    '    magic header not detected: bad magic number (at offset 0x0)'
}

After:

file:///Users/gbedford/Projects/test/test.js:212
    throw new ComponentError(variant22.val);
          ^

ComponentError: failed to extract interface information from component

Caused by:
    magic header not detected: bad magic number (at offset 0x0)
    at generate (file:///Users/gbedford/Projects/test/test.js:212:11)
    at file:///Users/gbedford/Projects/test/test.js:28:34

Node.js will display the object payload as enumerable otherwise for non strings.