Failures to archive test results are silently ignored

[h-arlt]

When downloading load test results, each configured agent controller is instructed to archive the results of its agents.
In case, such an archive could not be created for some reason, the agent controller logs a message at level WARN but does not signal this failure to the requesting master controller. As a consequence, the download continues as usual and succeeds in case no other issues occur.
In the end, the user might get incomplete test results without being notified about missing data.