Introduce new `structerr` package

Most of our codebase is nowadays using the helper.Err functions to format errors. While those are nice to propagate error codes across the codebase, they fall short to provide a more structured way of handling errors.

Right now we're between a rock and a hard place when it comes to providing context when an error happens:

- We can return a static error message. This has the benefit that
  the generated errors are brief and easily greppable, and because
  they are static it allows Sentry to properly deduplicate them. But
  the big downside is that you can now only log additional context
  via a separate log entry, and this makes it hard to correlate the
  errors with that additional log.

- On the other hand, we can try to add all information to the error
  message. While this now provides all the necessary context, it
  causes the error message to balloon in size. This both makes them
  hard to digest and trips Sentry.

Introduce a new structerr package to help solve this problem. Right now it really only does the same as the helper functions. But by using a new dedicated structerr.Error, we can iterate on its functionality and add additional information to this error. Eventually, this package will replace our helper.Err functions completely.