It seems that re-downloading qmlls is not possible when a download fails.
The qmlls download failed because of some network interruption and I couldn't redownload qmlls via the button because the current check thought that qmlls was successfully downloaded (even if the binary is missing).

Change the check to actually check if there is a qmlls binary, and allow to redownload qmlls binaries even when there is some leftovers from the last download lying around.