Add a friendly warning about the db_scheme for a Postgres database

[djvdorp]

This will give the user some headaches and breaks deployment for at least the uaadb and ccdb with pretty vague errors, another mistake we made ourselves.

[cfdreddbot]

Hey djvdorp!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/123433041

The labels on this github issue will be updated when the story is started.

[bentarnoff]

Thanks, @djvdorp !