Clearer error message if an invalid platform type is specified in config

[shantstepanian]

Current behavior:

Failed: java.lang.RuntimeException: Problem while logged in: com.gs.obevo.api.platform.DeployerRuntimeException: java.lang.ClassNotFoundException: PostgreSQL "

<dbSystemConfig type="PostgreSQL" sourceDirs="database">

The error message should indicate that a class was not found and print out the default built-in values for that