Reraise SlackClient rtm_connect exceptions

[mrbox]

It's copy&paste from #50 but including the change requested in PR

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PR Summary

Add possibility to re-raise exceptions happening when connecting to Slack RTM api

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[codecov]

Codecov Report

Merging #175 into master will decrease coverage by 0.43%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
- Coverage   64.16%   63.72%   -0.44%     
==========================================
  Files           9        9              
  Lines         293      295       +2     
==========================================
  Hits          188      188              
- Misses        105      107       +2

Impacted Files	Coverage Δ
slackclient/_client.py	41.07% <33.33%> (-1.53%)	❌

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[JustinAzoff]

This looks like a helpful change, but I would argue that the exception swallowing code in this function should simply be removed.

Looking at how this function is used:

https://github.com/search?l=Python&p=1&q=rtm_connect&type=Code&utf8=%E2%9C%93

I see two common patterns:

1. rtm_connect is called, and the return value is ignored.
2. rtm_connect is called, and if it does not return true, the program silently does nothing or an error message is printed.

In the first case, I'm not sure what happens when something like rtm_send_message is called immediately after that.

In the 2nd case, I see people doing various things like

print "Connection failed"
print "Connection Failed, invalid token?"
raise ConnectionError
raise Exception('Connection failed, invalid token ?')

In every single one of these cases this is a less helpful, and possibly completely misleading error message.

Yes, raising the exception by default would change the api, but as far as I can tell every single program that uses this library is already broken due to this exception swallowing.

[Roach]

We plan to refactor much of this library's error handling as part of an upcoming quality SDK project. As such, I'm going to close this in favor of removing the error suppression issues and cleaning up the exception logic.