TECH TASK: consolidate all "bad message" logic to one function

context

from discussion in keybase:

might be simpler to collapse all the corner case logic from handleNoCommand (which, like interveneIfBadMessage calls handleBadSubscriberMessage on a subset of logical cases) and stuff it all into interveneIfBadMessage at the top of the call to execute, thereby making the call to handleNoCommand (and the need for a default branch of the defacto switch statement that it provides) unnecessary, since if any executable makes it through to execute, it will either be one of the enumerated command types (in which case it will be handled by the switch statement) or it will be NONE, in which case it will be handled by the early-returning interveneIfBadMessage, in which case it will never hit the switch statement.

main motivation for this refactor: it is nice to start consolidating all the corner case logic in one place! we should finish the deal and not leave some of it before the switch statement and some of it after!