OPENDJ-2647 Schema problems in control-panel

The problem was due to an incremntal update of the SDK schema.
The problem does not appear when the SDK schema is updated in bulk and then built in one go.

SchemaLoader.java:
In getBaseSchema(), use a SchemaBuilder and create the server's schema with a newly built SDK schema.

Schema.java:
In the constructor, validate that the SDK schema has no warnings and throw a DirectoryException if it is has warnings.
This is more consistent with the rest of this class.

SchemaConfigManager.java, DirectoryServer.java:
Handled the newly thrown DirectoryException