Show warning if no lock can be aquired

[hagenw]

From time to time loading a dataset from a shared cache folder might get stuck for a long time (or for ever) as there is a leftover .lock file in the dataset. In some cases it might be valid, as another user started downloading the same dataset, but most often it happens due to leftover lock files, that couldn't be cleaned properly.

Thje biggest problem at the moment is that a user gets no feedback on what the problem is, and loading the data is simply stuck. We should try to avoid this by displaying that we are waiting to getting the lock to the folder and/or showing a warning message if the lock could not get acquired in 1 sec, that another user might be downloading the same data or that there is a leftover .lock file, which need to be manually deleted.