Fix error handling in replace-disks with new node

Currently the _CreateSingleBlockDev function only raises OpExecError and not
BlockDeviceError. This means that we don't release the instance's temporary
minors properly, and this creates problems later if the instance is removed
without master restart.

We could just use OpExecError, but adding it and leaving
BlockDeviceError in seems safer.

Reviewed-by: imsnah