Update call rejection should produce `UpdateCallRejectedError` instead of text error

[AndyGura]

Describe the bug Though agent-js declares error UpdateCallRejectedError, it cannot be caught in the wild, since initial http response from canister (always?) looks like this:

{
    "ok": true,
    "status": 202,
    "statusText": "Accepted"
}

And therefore if I write throw Error.reject or Debug.trap in canister, it will be processed by polling and produce pure text error here: https://github.com/dfinity/agent-js/blob/8254c47694c1286058a8293547bda98a6d19c8e1/packages/agent/src/polling/index.ts#L68-L73

I believe instance of UpdateCallRejectedError should be thrown instead for more convenient error-handling in top-level application

To Reproduce

1. Create and deploy motoko canister:

import Error "mo:base/Error";

actor class Test() {
  public shared func err() : async () {
    throw Error.reject("Rejected message");
  };
};

2. Create agent-js actor for it
3. Write test code:

try {
    await actor.err();
} catch (err) {
    console.log(err instanceof UpdateCallRejectedError);
}

4. Observe false in console and check that error contains only plain text message

Expected behavior UpdateCallRejectedError instance should be caught with all the appropriate fields in it.

[krpeacock]

I've reviewed this issue and added it to my queue. Thanks for reporting!