Detaching OS disk

**Related command**
$ az vm disk detach -g $RG--vm-name $VM --name $OS_DISK

**Describe the bug**
Command errors with `No disk with the name '...' was found`.

**To Reproduce**
$ az vm disk detach -g $RG--vm-name $VM --name $OS_DISK

**Expected behavior**
Users may not know that detaching an OS disk is not allowed. The cli should check if the disk specified is an OS disk and return a different error message saying something along the lines of `Only detaching data disks is allowed. '...' is an OS disk.`

**Environment summary**
xplat

**Additional context**
n/a