Add a check and more output to protect against invalid REPO_URLs

If you don't know that the url to git-repo itself can be overridden via
REPO_URL, it's hard to debug cases where REPO_URL is accidentally set to
another repository, e.g. inside a Jenkins CI job. What makes is even
harder is that the ".repo/repo" directory gets silently removed in such
cases as verifications fails, which makes it impossible to look at the
cloned files to understand the problem.

To better protect against such an issue, warn if the cloned git-repo
repository does not contain a top-level "repo" file, and state that the
".repo/repo" directory will be removed in case of a clone failure.

Change-Id: I697b4999205a5967910c0237772ccaada01e74d4