Add sentinel errors for the limiter package

[joeblubaugh]

The limiter#Limiter returns two kinds of errors from WaitN():

return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, burst)
return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n)

In the case of an HTTP request limit, I would ideally return a 429 Too Many Requests for each, but there are other errors that can be returned, like a context cancellation. It would be convenient to have an error type exposed in the package API that can be used with errors.As/errors.Is to see if the limiter has rejected the request.

[pracucci]

Makes sense. Feel free to open a PR about it.

[joeblubaugh]

When reading this code, I mistakenly navigated in golang.org/x/time/rate and read some of the WaitN implementation there. That's where the errors described above are generated. I think it wouldn't be a great practice for dskit to inspect the contents of those errors in order to return two different sentinel errors. I'll file an issue with that package instead.