* guix/packages.scm (&package-cross-build-system-error): New condition type.
(package-cross-derivation): Raise &package-cross-build-system-error
when the build system doesn't support cross builds.
* guix/ui.scm (call-with-error-handling): Add
package-cross-build-system-error? case.
* tests/packages.scm ("package-cross-derivation, no cross builder"): New test.