macOS allows re-install without requiring removal

A recent CB employee upgraded his personal laptop form 6.5 to  6.6.0 he did not properly uninstall macOS, the result was that nothing worked as expected and the installation was essentially 20% functional.

If couchbase employees can do this 100% sure potential customers experience this too.

An attempted overwrite/reinstall should do checking and at a least provide a warning to the end user.