Don't fail if unmounting the CMake disk image fails.

I don't know why, but sometimes it fails - the most recent failure was
due to it having already been unmounted!  Maybe the CMake installer
unmounts it some times but not others.  Just drive on if it fails.

Change-Id: I1b31356f08695b8d3b9e425f8475a422e1da0783
Reviewed-on: https://code.wireshark.org/review/3220
Reviewed-by: Guy Harris <guy@alum.mit.edu>