(bootstrapper) UX improvements: better error checking, messages

  * Restructured crisp-bootstrap to enable checks for existence and
    usability of specified program/mode/arguments before any comms
    transport is executed.

  * Added 5-second timeout on remote-program startup; after that, we'll
    assume it started without telling us and move on to the local
    program.

  * Made error messages in bluetooth-transport much more friendly.

  * Check for a bluetooth adapter first thing in bluetooth-transport, and
    complain at the user and exit if there isn't one.

  * Removed useless child-PID printout in unix-transport.