properly notify user when a match fails due to empty matching set

[naknomum]

attempt to fail properly when an empty matching set is used to match against. testing must be done, see (2) below.

PR fixes #524

:warning: this is only a draft because it might only fix part of the problem:

1. the code changes should properly short-circuit matching execution when an empty matching set is used (rather than requeue the job)
2. what is incomplete: will this properly set the task results such that the jsp page can report this back to user?