Difference between text and json error is hard to understand

[Qup42]

The difference between the following error cases should be made clearer in the UI:

• an error was returned by the server as QLever JSON
• an error was returned by the server with only a string as the response.

Most of the failures in this PR stem from this difference. Apart from the type (QUERY EXCEPTION vs UNDEFINED ERROR) (and some seemingly unrelated text changes) they look identical in the UI. This makes it very hard to understand the error from the UI.

[Qup42]

Extracting the error message from the JSOS is generally good, but makes the difference hard to spot in this special case.

• adding a note, that the message was extracted from the JSON could make this clearer
• notes what the different error messages mean/how they can occur could also clear this up.