* guix/ui.scm (print-unbound-variable-error): Add "error:" to the
message.
(report-unbound-variable-error): New procedure, with code formerly in
'report-load-error'.
(report-load-error): Use it.
(call-with-unbound-variable-handling): New procedure.
(with-unbound-variable-handling): New macro.
* guix/scripts/build.scm (options->derivations): Wrap body in
'with-unbound-variable-handling'.
* tests/guix-build.sh (GUIX_PACKAGE_PATH): Add test.