bug#36592: no output for guix package -f when the file evaluates to nothing

[Julien Lepiller]

Users tend to copy what they see in our code base, so it's not rare to have to 
help them realise that if their last statement is (define-public ...), the file 
doesn't evaluate to a package.

Running guix build -f mypackage.scm at least gives an error saying it doesn't 
know how to compile #<unspecified>.

Running guix package -f mypackage.scm returns immediately with no error message 
and a status of 0, but doesn't actually build or install anything. You can even 
try with an empty file :)

There should be an error message, and probably even a hint.