Improve error message for invalid state operations

[SimonWoolf]

Currently, a publish when the library is closed, or the channel is detached or failed, all return "channel operation failed (invalid channel state)". This isn't the most helpful when customers complain thhat they're getting that and want to know why -- it could be a bit more specific (because connection is ' + connection.state, 'because channel state is ' + this.state`), and in the case of failed it could perhaps return the stringified errorReason too.

┆Issue is synchronized with this Jira Story by Unito

[ably-sync-bot]

➤ Marat Alekperov commented:

This issue was done shortly after it was reported but somehow remained opened.