Falkor DB Query Engine: Add delete method and handle exceptions

[AgentGenie]

Why are these changes needed?

Some exceptions are not handled in explicit way. Handle those that the APIs will throw meaningful message.
Add delete API.

Related issue number

Follow up on #177

Checks

tested with notebook example locally.

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[marklysze]

Works well, thanks!