Add API Error handling and local rate-limiting

[hmmwhatsthisdo]

This PR covers two separate, yet two related issues:

• PSSlack does not gracefully handle errors from the Slack API (Some of these come with 4xx/5xx HTTP status codes, meaning Invoke-RestMethod throws an exception)
• PSSlack does not respect Slack's API rate-limiting (see Rate-Limiting is not handled gracefully #52)

By adding in a basic error-handler on the API - one that handles both 4xx/5xx errors from the API as well as results where ok is $false - we can handle rate-limiting a little better.

Local rate-limiting has been implemented behind a switch (-RateLimit) on Send-SlackApi. This uses the Leaky Bucket algorithm to allow for 25 burst requests, with 1 second between requests otherwise. This seems to be very similar to Slack's rate-limit parameters, and will automatically recover from rate-limit errors returned by Slack's API.

This PR should resolve #52.

[RamblingCookieMonster]

Awesome! At a quick glance, don't see any blockers - will poke around tomorrow or this weekend and merge it in.

At some point, it might be handy to:

• Add the param to other functions that might trigger rate limiting, pass on to send-slackapi
• Add a RateLimit option to Set/Get PSSlackConfig

Thanks again, this will be quite handy!

[hmmwhatsthisdo]

Add the param to other functions that might trigger rate limiting, pass on to send-slackapi

Right - now that Remove-SlackMessage has been merged in, I suppose it'd be a good idea to set it on that function.

Add a RateLimit option to Set/Get PSSlackConfig

I briefly considered this - however, I'm unsure what would be best here from a usability perspective. Perhaps a config flag to globally disable rate-limiting?

[RamblingCookieMonster]

Alrighty, ran through again, this looks good to me, and no one else has chimed in, merging! Thanks again for taking the time to add this!

[RamblingCookieMonster]

I briefly considered this - however, I'm unsure what would be best here from a usability perspective. Perhaps a config flag to globally disable rate-limiting?

Not sure on specifics. Maybe:

• A -RateLimit bool or switch - reflects same parameter name
• Default to not present, allowing us to use it as default value for functions that are affected by rate limiting
• Avoid as default on Send-SlackApi, unless we know it applies to everything Send-SlackApi can hit

Open to other options though - cheers!