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.
f304c9c237