Add option to detect unmapped error

[mszostok]

Description

This pull request adds:

• dedicated type error for UnmappedError
• unit tests coverage

Reason

The library uses the UnmarshallingErrorEvent error in two cases:

1. when there was a real json.Unmarshal
2. or when there was no corresponding Go struct for unmarshal process

We want to be able to check what kind of error we got. As a result, we will be able to do the assertion whether we care about this unmarshal error or not. Example snippet:

// handlingEvents holds event names for which we have corresponding business logic.
var handlingEvents = sets.NewString("message", "connected")

// ...trimmed...

switch ev := msg.Data.(type) {
case *slack.UnmarshallingErrorEvent:
  err, ok := ev.ErrorObj.(slack.UnmappedError)
  if ok && !handlingEvents.Has(err.EventType()) { // it's not an err for us, as we don't care about this event type.
    b.log.Debugf("Slack unmarshalling error: %+v", ev.Error())
    continue
  }

  b.log.Errorf("Slack unmarshalling error: %+v", ev.Error())
}

Currently, we have in logs:

ERRO[2022-06-15T11:30:02Z] Slack unmarshalling error: RTM Error: Received unmapped event "user_status_changed"

We could do the error string assertion but it's not a good practice, so we want to avoid that.

Implementation
I decided to leave the UnmarshallingErrorEvent as it is, but instead change

err := fmt.Errorf("RTM Error: Received unmapped event %q", typeStr)

to typed error. As a result, is not a breaking change.

[kanata2]

I commented on one point, but LGTM.

[kanata2]

👍