Change BlockDev.Remove() failure result

Currently, the Remove() methods of block devices return True/False.
This doesn't permit any error detail reporting.

This patch changes the return type to None for success, and raises
BlockDeviceError in case of failure. This permits the details to be
passed up the stack.

The patch also simplifies a little the Remove method of file-based
devices (no stat first, just try unlink).

Reviewed-by: ultrotter