"Error: system is not ready for upgrade" is not helpful

[kparal]

We're getting a lot of feedback from people who try to run dnf system-upgrade reboot and get

Error: system is not ready for upgrade

I think that often happens when you perform some other dnf operation after downloading the packages, which invalidates them (and rightfully so). But the error itself is not very helpful, people are confused and don't know what to do. Please make it more explanatory. What about:

Error: system is not ready for upgrade. This often happens when FIXME. Please try to start system-upgrade procedure from scratch (downloading the packages first).

[jwaldmann]

"(downloading packages first)" - is there not some caching mechanism that avoids redundant downloads? see issue #22