Fixed bug where putting down a running drive crashes tapeserverd
Vlado found this bug whilst running tapserverd with real tape hardware. Mounts last long enough with real hardware for an operator to easily use "tpconfig down" on a running drive. The tapserverd process was not handling the WAIT_DOWN state correctly and was throwing an unexpected exception that in turn caused the process to crash due to the resulting abort signal. The tapserverd process was not handling the WAIT_DOWN state correctly due to some over simplifications I made to the architecture of tapeserverd.