Enable the plugin system to extend the `helpers` passed to tasks

[benjie]

Also: wrap task executors (e.g. for shared error handling logic).

[mike-ayres]

does shared error handling logic include how the error is parsed for the database? I have a generic name/message for my custom errors and the relevant data is in other error properties. I'd like for all that data to go into the database the way I can control how it displays in the logs.

If it's different, I can make a new issue and perhaps take a stab at implementing such a thing.

This is the bit of code that I'd like to be overwritten with a custom parsing function:

const { message: rawMessage, stack } = err;
                /**
                 * Guaranteed to be a non-empty string
                 */
                const message = rawMessage ||
                    String(err) ||
                    "Non error or error without message thrown.";

[benjie]

The above is meant to enable something like wrapping all task executors, e.g.:

const plugin = {
  worker: {
    middleware: {
      defineTaskExecutor(details, next) {
        const oldExecutor = details.executor;
        // Wrap the executor in error handling
        details.executor = async (...args) => {
          try {
            return await oldExecutor(...args);
          } catch (e) {
            await doStandardErrorHandling(e);
            return null;
          }
        };
        return next();
      }
    }
  }
};

Nothing more than that.