Can't change harts during operations, and the current hart becoming...

Can't change harts during operations, and the current hart becoming unavailable may terminate the abstract command with error. (#322)

* Can't change harts during operations.

See #315.

* Only restrict hartsel changes during abstract cmds

reset/halt could take a very long time if a hart is asleep, and we don't
want to prevent debugging other harts in that case.

* Set cmderr for hartsel change during abstract cmd.

* Clarify reset and hasel during abstract command.

* Incorporate review feedback.

* Make debuggers handle harts becoming unavailable.

Instead of having the DM correctly handle a hart that becomes
unavailable, make debuggers responsible for that. This seems like a good
tradeoff because this should be rare.

* Clarifications requested in review.

* Clarify per review.