Fine grained state and propagation

[baranowb]

Right now FSM of BisectablePullRequest is one that drives CRYO. Its a bit fragile and aggressive( state propagates up and down the tree if case of error) - its simple and effective but not very good.
This should be refined to compartmentalize state to single BPR( rather than tree). Secondly there should be a way to access this from roots, since roots is what CRYO operate on( tree is recursive).
Second issue is a bit of child of first one. If something goes wrong in tree of BPR we get whole tree with agressive state change, which tells us nothing to pinpoint fault( unless you go through logs, which is less than ideal).

So:

1. Refine state of BPR
2. Refine how its accessed from roots
3. Improve fault report either with some steps or just via above.