scheduling / abort and UI tweaks

This pull request contains 4 separate changes - but they all build on each other (with the exception of plugin help)

1) fixed issue 15966 - if a build cannot be scheduled don't barf
2) if a flow is aborted then abort any downstream jobs that are in progress or queued
3) small ui / english tweaks (link a flowCause back to the job flow)
4) allow plugins to contribute to the DSL help