derivations: Raise an error for references to non-existent outputs.

derivations: Raise an error for references to non-existent outputs.

Fixes <http://bugs.gnu.org/19630>.
Reported by Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de>.

* guix/derivations.scm (&derivation-error,
  &derivation-missing-output-error): New error conditions.
  (derivation->output-path): Raise a '&derivation-missing-output-error'
  if OUTPUT is not an output of DRV.
* guix/ui.scm (call-with-error-handling): Add case for
  'derivation-missing-output-error?'.
  (show-what-to-build): Check whether (derivation-outputs drv) is
  empty.
* tests/packages.scm ("reference to non-existent output"): Add test.