improve download completion logic

Several things can cause downloads to fail. Presently, one common problem is expiring SB authentication. This should be fixed by #131 (closed), but other issues can still arise. The 00_get_data.Rmd script contains several blocks where completion is tested by the existence of a given directory or file. When several files are in one of these blocks, a failed download can cause some of these files to be skipped the next time the pipeline is run.

See for instance here, where only the reference catchments are looked for. If the reference catchments are present but not the reference flowlines, the flowlines would be skipped.

Blocks such as this should be reworked so that each necessary file is tested for individually.

CC @mewieczo