Handle missing or no rainfall rasters

[gassc]

A user encountered an issue where the rainfall download tool rain successfully, but generated an empty JSON file. The culvert calculator ran too, but without rainfall rasters, all peak flow estimates were empty. The telltale sign (other than the empty output) was that the processing time for calculating average rainfall per catchment per storm frequency was 0. None of that should have happened.

This should be handled:

• as an input validation step in the Python toolbox (#14)
• in the tool itself: we shouldn't run delineations if there aren't any rainfall rasters to reference, and we shouldn't kick things off if the JSON file is empty.

[gassc]

We also need to investigate why the rainfall download tool is not downloading anything. If it doesn't, it should obviously fail too.