Frontend should show a explanation when `/messages` returns a single message because the others were compacted away

[rikimaru0345]

Example scenario: The user wants to see the last 100 messages, but the backend returns only one message, because the requested range (latest .. latest-100) has been compacted.

In that case the frontend should show some sort of message to explain what is going on; why only a single message is shown.

Similar to #25, which is also about handling of error-/edge-cases.

[weeco]

~~In that sense we could reiterate how we want to collect messages on the /messages endpoint. I'd say the user's expectation is to get 100 items back when he specifies that as itemcount.~~

~~The current implementation of the endpoint will likely return less results on compacted topics, especially when you start at old offsets.~~

Update: This has been reworked now - though the issue is still valid. The backend already exposes a isDrained boolean for each crawled partition that indicates whether there were more messages available or not.