Add handling for HTTP 429 responses

[cuu508]

When pinging a check via slug, if the request gets rate-limited, Healthchecks.io can return HTTP 429 response: https://healthchecks.io/docs/http_api/#success-slug

When this happens, runitor outputs:

2022/10/17 13:15:06 Ping(start): max tries reached after try 3. last error: <nil>

It would be nice if runitor would print a more descriptive "rate limit exceeded" message here.

Note: this only happens when pinging by slug. When pinging by uuid, Healthchecks.io always returns HTTP 200, even in rate-limited cases, for backwards compatibility.

Note 2: Healthchecks.io (the hosted service) has rate-limiting, but the open-source Healthchecks project does not. For efficiency, the rate-limiting is done by nginx, so there's no sign of it in the python code.

[bdd]

This was a bug.
Added the commit above. Will cut a release soon.

[bdd]

I released v1.0.0 🎉 including this bugfix.