Improved error reporting and dependency checks off by default

This pull request adds checks that the required environment variables PUBLISHER_LOGIN and PUBLISHER_PASSWORD has been set correctly. If not, the missing settings are reported in an understandable way. In addition to that, this pull request also ensures that the 3.0.3-rc1 version of the Geppetto Publisher is used. In that version, the cross-reference and dependency checks are turned off by default.