Convert IOErrors for /proc/drbd into our errors

If /proc/drbd can't be opened, this raises an IOError, but all the
error-handling behaviour in backend treats only BlockDeviceErrors. This
creates a plain failure in cluster verify and in other RPC calls.

This patch simply converts EnvironmentErrors into BlockDeviceErrors, and
also changes the RPC result for NV_DRBDLIST and its handling to be able
to show the error. The other RPC calls work by default now, due the
existing error handling.

Reviewed-by: ultrotter