Suggest explicit dbpath when starting mongod on macOS and the default dbpath isn't found

macOS Catalina runs in a dedicated, read-only system volume. Consequently, the default dbpath of "/data/db" is not able to be created. This issue is discussed in more detail in https://stackoverflow.com/questions/58283257/mongodb-cant-find-data-directory-after-upgrading-to-mac-os-10-15-catalina

Since this default will not work on Catalina and later, we should suggest an explicit dbpath be specified when starting mongod on macOS.