The script scripts/pull_github_pr.sh begins by fetching some information
from github, which can cause a noticable wait that the user doesn't
understand - so in this patch we add a couple of messages on what is
happening in the beginning of the script.
Moreover, if an invalid pull-request number is given, the script used
to give mysterious errors when incorrect commands ran using the name
"null" - in this patch we recognize this case and print a clear "Not Found"
error message.
Finally, the PR_REPO variable was never used, so this patch removes it.
Message-Id: <20200923151905.674565-1-nyh@scylladb.com>