Show error symbol when watch is running

Currently it is hard to see that error has happened especially when log panel is collapsed while watch task is running.

Here comes the use case: I have a watch task which performs tslint / jshint / whatever similar which is validating something and returning errors when problems occur. I change the file, error happens, but I don't see it and my build has failed. If the logs are shown I still see the error message, so it is kind of OK, but when it is collapsed there is no way to see that error happened.

Would be nice to also track watch events for errors and display the error symbol.

Start by tracing how watch-task events are handled and how the log panel displays its status. The work is done when an error from a running watch task is represented by an error symbol even while the log panel is collapsed.

Written by the indexing model from the issue text.