Improve error message if error response has resonse_metadata

[vbanthia-zz]

Some slack errors responses such as dialog.open also contains response_metadata field with more useful information related to error. This PR will improve error message by showing that in client error message.

Before

Slack::Web::Api::Errors::SlackError - validation_errors

After

2018-01-29 17:51:56 - Slack::Web::Api::Errors::SlackError - validation_errors Response Metadata: [ERROR] Element 0 field `label` cannot be longer than 24 characters:

https://api.slack.com/methods/dialog.open

[dangerpr-bot]

	2 Warnings
⚠️	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#201](https://github.com/slack-ruby/slack-ruby-client/pull/201): Improve error message if error response has resonse_metadata - [@vbanthia](https://github.com/vbanthia).

Generated by 🚫 danger

[dblock]

I think this shows that we did a minimum job in the response handler here earlier :) Let's add a response_metadata field (and possibly others) to SlackError, assign the response itself to the class as a field and expose anything useful. Then that class should implement to_s and combine all the errors there.

Everything needs tests, too.

[vbanthia-zz]

Will do over weekend.

[dblock]

Bump @vbanthia ?

[henrik]

@vbanthia, just a friendly ping :) I know how life can get in the way.

[dblock]

@henrik maybe you want to continue this PR?

[henrik]

@dblock I was considering it! Just ended my lab day, but I might do some more Slack work next time around, and if so I'll see :) If anyone else wants to do it, please feel free.

[dblock]

I'll close this. Feel free to reopen if traction.