More robust Topicmaps REST API

[jri]

... in case of errors, e.g. when

• adding a topic/assoc to a topicmap which is already added. Currently the topicmap gets corrupted in this case
• setting view properties for a topic which is not in the topicmap. Currently a non-informativ NullPointerException? is thrown.

[Jörg Richter <jri@…>]

In b226ab6e84cd1ddf7a8899f11ffba29bab30bb8a/deepamehta:

Topicmaps fix: ignore add if added already (#800).

If adding a topic/assoc to a topicmap which is added already an exception is thrown. The topicmap is not corrupted.

Topicmaps API:

1 new method:

boolean isAssociationInTopicmap(long topicmapId, long assocId)

See #800.

[Jörg Richter <jri@…>]

In e12df480003d1188d199791585ae989ae56ddd4e/deepamehta:

Topicmaps: more informative error messages (#800).

... when these operations fail:

• Setting topic view properties
• Removing association from topicmap

See #800.