Checks creds before uploading and running presubmit, generalizing
the case of Rietveld. If they are missing, suggests a URL to
generate them.

R=andybons@chromium.org,phajdan.jr@chromium.org
BUG=583153

Review URL: https://codereview.chromium.org/1882583003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299883 0039d316-1c4b-4281-b951-d872f2087c98