work on tickets

 - resilience against missing / incimplete VE and intermittent pip/easy install errors
 - introduce cleanup flags to bootstrapper
 - take care of race conditions in backfilling scheduler, on failing / finishing pilots
 - fix some layout issues in plots
 - better error reporting in unit manager and backfilling scheduler
 - simplify and speed up pilot state checks
 - add max reconnect-fail count to pilot state checker