File Support

For the File plugin, the feed_url points to the manifest file, rather than the directory. Other plugins use the directory, so it is likely that others will make this mistake also.

WRONG:
feed_url = https://repos.fedorapeople.org/pulp/pulp/fixtures/file/

RIGHT:
feed_url = https://repos.fedorapeople.org/pulp/pulp/fixtures/file/PULP_MANIFEST

This issue is to impove the exception handling, because it was tricky to debug.

Some notes:

• os.path.basename returns "" when passed a string that ends with "/"
• os.makedirs ignores the working directory when passed a string that starts with "/"
• the combination of ^ means that using WRONG feed_url results in a PermissionsError, trying to write to /pulp/pulp/fixtures/file/