model: print bug reports at end of each execution

This ensures:
- that bug reports are printed only at the end of feasible executions
- that execution summaries are printed only for complete (or halted and
  buggy) executions that are feasible
- that all bug reports will have the same formatting (at least, ones
  that use the 'assert_bug()' function)

Note that deadlocks are the only bugs reported this way right now. I'll
fix that up soon.