Add indicator of datasets that are missing one of the requested files to the ui

[lukasjelonek]

Currently the ui prints the number of completed and failed jobs, where failed jobs denotes everything that had a network error and completed denotes everything that succeeded.

Bakrep contains some incomplete datasets, i.e. datasets that don't have all expected outputs associated. Currently downloading missing files from these datasets is counted as completed. The result is confusing. In the end the number of completed downloads might be higher than the actual number of files present in the download folder.

To improve this situation I propose to add a third counter to the ui output: partial Datasets. This counter counts all datasets that where available on the server, but were missing at least one of the requested files.