refactor: rework of the Manager events

- rename "connect_error" to "error"
- remove "reconnecting" (duplicate of "reconnect_attempt")

The updated list of events emitted by the Manager:

- open:                successful (re)connection
- error:               (re)connection failure (previously: "connect_error") or error after a successful connection
- close:               disconnection

- ping:                ping packet
- packet:              data packet

- reconnect_attempt:   reconnection attempt (previously: "reconnect_attempt" & "reconnecting")
- reconnect:           successful reconnection
- reconnect_error:     reconnection failure
- reconnect_failed:    reconnection failure after all attempts

For reference, the Socket instance emits the following events:

- connect:             successful connection to a Namespace
- connect_error:       connection failure
- disconnect:          disconnection