binary backend always claims to work if devportal-metadata.json is present

A common failure mode of pkgme-service is "NoBinariesFound" which we know
to mean that pkgme-binary couldn't handle the submission, but it's not immediately
obvious.

If the binary backend is going to fail if there are no binaries, then it could check
this in the "want" script, rather than only looking for devportal-metadata.json.

That would mean that where pkgme-service couldn't handle the submission the
error would be "No suitable backend", which is much more obvious (and could even be
caught and transformed in to something even more explicit by pkgme-service
with no layering violation.)

Thanks,

James