Make the error messages about downloading STI scripts more friendly

Give users more friendly message when they are trying to use `sti build` with non-STI image and without `-s` flag.
Additionally don't replace the download error with custom error, but return the original error to users.
I assumed that when the `downloads` is empty, it means there was no download at all (no scripts). Then if also the defaultUrl is empty, it means the build was executed with non-compatible image and without `-s`....