Warn user of failed transitions

For today, as I understand, if preconditions are not met for issue transition from, for example, Code Review to Testing(Merged Pull-Request), there is no way to figure it out.

No doubt, you can look up the corresponding entry in the Activity flow, but this is too unconvenient. It would be much better to get notified about failed transition right in Stash when performing any of Pull-Request actions which trigger such a issue transition