Handle the case when watcher returns before the channel is closed

#### Motivation / Context

Currently, the watcher is started when a channel is opened (see `newCh` function in `session/channel.go` and assumed that it will only return after the channel instance from go-perun is closed. So we just log the error.

This assumption holds good for all our current tests cases, but the cases where the assumption will fail also should be handled. #### Description

Explore when this assumption can fail and handle the error for such cases. One possible scenario where it fails could be when connection to blockchain fails.

##### Relates to