fixup! Refactor OnSerialNumber behavior

Signed-off-by: Luke Stuyvenberg <stuyvenberg@wisc.edu>
Remove unused/unnecessary member functions

The stage adapter was first written a while ago, presumably when some of
the convenience of the base classes was not yet in place. As a result, we
have a lot of member functions overridden that we needn't.

Signed-off-by: Luke Stuyvenberg <stuyvenberg@wisc.edu>
Don't disable 'unreferenced parameter' warning...

...instead, don't name the parameters. This seems to be the preference of
the MM team (see MMDevice/DeviceBase.h and others), so we will follow
suit. (Besides, a global warning disable is rarely a good sign.)

Signed-off-by: Luke Stuyvenberg <stuyvenberg@wisc.edu>
Improve/normalize error reporting & return values

Since parts of this code were written at very different times, significant
portions use very different return values. I've normalized these.

More importantly, though, I've used specific error messages wherever they
applied. In particular: unsupported operations return
DEVICE_UNSUPPORTED_COMMAND, things we intend to add return
DEVICE_NOT_YET_IMPLEMENTED, issues with PiUsb connectivity return
DEVICE_NOT_CONNECTED.

(Hopefully, all the DEVICE_NOT_YET_IMPLEMENTED messages will disappear two
days from now. ;-) )

Signed-off-by: Luke Stuyvenberg <stuyvenberg@wisc.edu>