Hi Guix!

this patch adds a hint to guix build, when you try to build from a file
and it returns #<unspecified>.  The most common error is when you copy
the structure from other guix modules and use `define-public` for your
package. The file returns #<unspecified> and you get this message:

> #<unspecified>: not something we can build.

Which is not very helpful. Here is what happens with this change:

> guix build: error: #<unspecified>: not something we can build
> 
> hint: If you build from a file, make sure the last scheme expression
> returns a package value.  `define-public' defines a variable, but
> returns #<unspecified>.  To fix this, add a scheme expression at the
> end of the file that consists only of the package's variable name you
> defined.  ex:
> 
>      (define-public my-package
>        (package
>          ...))
>      
>      my-package

If instead, you return a value that is not understood, the message is
simpler (as this is not something that happens out of confusion / being
new to guix or guile):

> guix build: error: 1: not something we can build
> 
> hint: If you build from a file, make sure the last scheme expression
> returns a package, gexp, derivation value or a list of such values.

(here the file returned "1" which we can't build :))