[rbdemo] Add some checks to prevent us from breaking the demo site.

Review Request #6338 — Created Sept. 19, 2014 and submitted — Latest diff uploaded

Information

rb-extension-pack
2bae2dc...

Reviewers

This adds some pre-reset checks to rbdemo's reset-demo command that
ensures we have all the proper permissions and abilities we need prior
to resetting the demo database state. This includes read-access to the
fixtures and media directories, write access to the destination media
directory, and the ability to change ownership. It also validates that
the specified user and group are valid.

Tested each possible validation failure and saw that I got appropriate
errors.

Tested with a valid setup and saw it reset the demo database.