Deleting types when instances still exist

[jri]

The (API) user must not be able to delete types when instances of that type still exist. Instead an error message must be generated.

Currently it is possible e.g. in the Webclient to delete a type when instances still exist. The consequence is a non-consistent DB.

[Jörg Richter <jri@…>]

In 2d6fe77eeaa65d5f211bf1359391a509d9761e4a/deepamehta:

Core fix: deleting types when insts exist (#789).

A type can't be deleted if instances still exist.
If tried an exception is thrown.
Applies to topic types and association types.

Pending: reporting the error to the user.

See #789.

[jri]

Meanwhile the error is reported to the user through #790