The qlever start command should stop in case of error

[ludovicm67]

Using version 0.5.18 (and it was already the case on the previous ones), it seems that the qlever start command is not giving the prompt back to the user in case of an error.

A simple way to reproduce this, is to try to start a QLever instance without having the index generated.

Running qlever start displays the error, but doesn't stop the execution.
That means, it's complicated for shell script to know that an issue occurred, as the prompt is not given back to the script, and no relevant exit code is returned.

It's possible to listen the logs for "ERROR" or so, and then force the kill of the process, but the issue with this is that it might also kill the process in case a user tries to run an invalid request.