To keep archive creation implementation simple and prevent any
confusion, we decided to remove `--keepurl` flag and embed URL
directly without downloading the file from it . In this way, we can
ensure consistent behavior in either package creation or spec file
creation. Also, it increases the portability of spec file.
Before this PR, CLI functions fatal out when encountering error
instead of returning it. Such behavior makes it hard to reuse
the functions nor writing unit tests. This PR aims to make functions
return errors instead of error out.
