Generate errors for the FlowMods

[ajoaoff]

Original issue opened by @renanrodrigo at kytos#23.

Such as malformed flow JSON or inexistent datapath

[viniarck]

@ajoaoff on PR #52, that made available the following events:

kytos/flow_manager.flow.removed
kytos/flow_manager.flow.added

and, in case of connection errors (in addition to the OFPT_ERROR that were previously covered), it'll send a kytos/flow_manager.flow.error PR #60.

Regarding the validations, we've got on our radar to handle on issue #43, so I think we can close this issue and handle the rest, that is the validation on this issue, wdyt? Once your PR on mef_eline that addresses OAS and overall validation strategy, we can implement a similar approach when fixing the issue 43