v1: github folder metadata should 404 if its a file
v1: github folder metadata should 404 if its a file

If we give _metadata_folder a folder path and it finds a file at that
path, it should throw a 404 Metadata error.

Johnetordoff/waterbutler,CenterForOpenScience/waterbutler,RCOSDP/waterbutler,rdhyee/waterbutler,kwierman/waterbutler,felliott/waterbutler,TomBaxter/waterbutler