app::zaberLowLevel doesn't halt on missing stage

[Jared R. Males]

or otherwise handle it. If a configured stage isn't found, which can happen if say the stage is in local control or disconnected, zaberLowLevel silently continues but somehow ends up trying to communicate with the missing stage. This causes timeouts and eventual crashes.

[Jared R. Males]

there is a check for unknown stages found that aren't configured, but no check for a configured stage not found. it appears that in this case zaberStage::m_deviceAddress will be a random number. We should initialize this to something and check it, and prevent communications in this case.

[Jared R. Males]

also need to add the check for configured not found.

[Jared R. Males]

checks for missing stage implemented in d7fc194. Needs to be tested with actually missing stage. Can be done by disconnected stagebs

[Jared R. Males]

tested.