V2: Throw errors in ReflectMessage instead of returning them

[timostamm]

The reflection API provided by ReflectMessage, ReflectList, and ReflectMap validates a value before setting it. The methods return an error for an invalid value. For example:

import { UserDesc } from "./gen/ts/extra/example_pb.js";
import { create } from "@bufbuild/protobuf";
import { reflect } from "@bufbuild/protobuf/reflect";

const user = create(UserDesc);
const msg = reflect(UserDesc, user);

const err = msg.set(UserDesc.field.firstName, 123);
console.log(err.message); // expected string, got 123

The firstName field is a string, and calling msg.set returns an error.

While this pattern works well, returning errors is very uncommon. This PR changes the behavior to throw errors instead. To handle the error, the example above changes to:

const user = create(UserDesc);
const msg = reflect(UserDesc, user);

try {
  msg.set(UserDesc.field.firstName, 123);
} catch (e) {
  if (isFieldError(e)) {
    console.log(e.message); // expected string, got 123
  }
}

[timostamm]

We could throw in checkField, but the function is internal, so we can punt on it until there is a really good reason do change the function.