2 kinds of errors (or warnings) that would assist workflow debugging

[nschneid]

It would be nice if workflow analysis reported two kinds of issues:

1. When a plan specifies an invalid branch for a branch point: I had (BranchPoint: 1) in a plan even though BranchPoint had no branch called 1; ducttape merely said no vertices were found in the plan.
2. When a branch graft specifies branches on irrelevant branch points: I had an input like x=$y@z[BP1:0,BP2:0] where the z task was associated with BP1 but not BP2. This caused the desired realization (with [BP1:0]) to be filtered out silently.