[Fix][cherry-pick] Handle error when switch is not connected

[viniarck]

Fixes #26

I cherry picked commits from PR #47 to use the base branch as fix/overlapping_flows since PR #45 might get on hold depending on our priorities, and PR 47 was on top of PR 45, so this unblocks me to continue on other tasks.

Description of the change

• Raises SwitchNotConnectedError if the switch is not connected and return 424.
• This PR depends on this PR on kytos core, just so the connection is no longer None.

Notice that as mentioned on the dependent PR, there's still cases that we're not handling when the socket TX fails, flow_manager willl need to start listening to kytos/core.openflow.connection.error, but notice that this isn't also being propagated to via the rest request because that's sync, so for clients that need to handle that either they'll have to send an async KytosEvent for kytos.flow_manager.flows.(install|delete) when we have implemented full error handling, or they can rely on the force option that we'll have on issue #46, but that will likely take a lot longer on average - which might or not be a concern - so it'll depends on the responsiveness that the client needs.

So, now if a switch isn't connected at the time when the flow mod is about to be sent, clients will get:

{
    "code": 424,
    "description": "switch 00:00:00:00:00:00:00:01 isn't connected",
    "name": "Failed Dependency"
}

[italovalcy]

The changes look good to me. Thanks for adding a handler for this error, @viniarck .

[viniarck]

The changes look good to me. Thanks for adding a handler for this error, @viniarck .

Thanks @italovalcy and @ajoaoff for your reviews on this PR. This PR is on topf of this one PR #44, I'll merge this one here once PR 44 is reviewed again, if either of you could do a second pass on that on that'd be appreciated.