Improve error message for Source URLs lacking tarball name

[chorsley]

I was using rpmbuild to create Python RPMs based on a local tarball. I would receive this somewhat cryptic error:

error: File /home/makerpm/rpmbuild/SOURCES: Is a directory

After digging through the rpmbuild code for hints, it turned out that where py2pack (which I used for generating spec files) produced a spec file header of:

Source: https://files.pythonhosted.org/packages/source/m/mixbox/

I needed:

Source: https://files.pythonhosted.org/packages/source/m/mixbox/mixbox-%{version}.tar.gz

It was not obvious to me that the name of the tarball in fact needed to be in the external URL when I was building everything locally. I'd like to suggest a more explicit error if the version / tarball name cannot be extracted from the Source field.