At the moment, it's possible to get MC into this situation:

* NewChannels
* a handler H0 with BypassApproval=FALSE exists
* a handler H1 with BypassApproval=TRUE exists
* MC decides to bypass approvers
* H1 exits
* MC runs handlers
* H1 doesn't exist any more
* H0 is run without approval

or even this:

* NewChannels
* a handler H0 with BypassApproval=FALSE exists
* a handler H1 with BypassApproval=TRUE exists
* MC decides to bypass approvers
* H1 exits, but MC hasn't seen it disappear yet
* MC runs handlers
* H1 is run; calling its HandleChannels method fails
* Only now does MC see that H1 has exited
* H0 is run without approval

We should convert dispatching to have an extra phase: before running approvers, try to run BypassApproval handlers in preference order. If one of them succeeds, stop dispatching; if they all fail (or there aren't any), continue to approvers.

This is likely to require refactoring in the dispatcher code, and is probably too intrusive for 5.2.