Better handling of erroneous origins submitted to save code now

Despite clear instructions to submit to save code now the exact origin from which a checkout/clone can be performed, several users keep submitting erroneous origins. A typical example is https://github.com/TIBHannover/ that points to an "organization" on GitHub with some 80+ projects under its umbrella, and then, rightfully, fails.

In the current setup, the user that originates such erroneous request does not get any feedback about the mistake (unless she looks up the result proactively later on), and the "save code now" is wasted.

It would be desirable to provide the user with feedback that helps fix the issue.

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Migrated from T3252 (view on Phabricator)