Add mode to propagate the error from Pipeline to the foreground thread

[mthrok]

By default the errors happening in Pipeline is not propagated to the main thread. This is by design, but it is annoying when building a pipeline. (It is not straightforward to catch issue in tests unless you are watching the log.)

An idea to solve this is to add a sort of "strict" mode people use while development. In this mode, the Pipeline reacts to a task error and send some signal to the foreground thread.