planemo shed_update should abort if uploading no files

In general, I think attempting to upload zero files to the Tool Shed with ``planemo shed_update`` ought to be an error.

In the specific case of a ``tool_dependency_definition`` (as in example below), if the upload bundle is not exactly one file named ``tool_dependencies.xml`` then ``planemo shed_update`` ought to abort with an error.

Background: I think that #624 aka https://github.com/galaxyproject/galaxy/issues/3546 came about due to me having accidentally upload an empty bundle to the (Test) Tool Shed. Due to a human error, I ran planemo shed_update on https://github.com/peterjc/pico_galaxy (rather than https://github.com/peterjc/galaxy_mira where the code has moved), but still had orphan .shed.yml files present for the MIRA packages - now fixed peterjc/pico_galaxy@65b8925